This is called regression. It is common for a toddler to regress to a previous developmental stage when a new baby joins the family. This does seem extreme. There may be more involved than just a new baby. As you said, he may be doing it to get attention. If so, he needs more attention.
Are you paying attention to him while you're taking care of the baby and not just when the baby is asleep? Do you routinely ask him to help by asking him to hand you a diaper or go get the pacifier or whatever it is you need that he can get or do, even tho it's easier for you to get or do it? Do you talk with him as you're changing the baby's diaper?
Have you talked with him about how he feels? You can suggest that having a new baby sister is a bummer and you understand if he's angry, scared, upset, etc. There are some good books written for toddlers about their feelings as well as about getting a new sibling. You could try reading some of those with him. Ask him questions as you read. Ask him specific questions as well as more open ended ones such as "what do you think?"
As important as talking with him is accepting whatever he says and accepting his feelings. For example, he may say he hates the baby. Tell him it's OK to hate her. That you know some day he'll feel differently.
Let him know that you know and accept that this is a difficult time for him and ask him how you can help him. Ask him if he'd like to sit on your lap while you read to him. Have him sit next to you while you feed the baby. Do everything that you can think of that will let him know that he's still special. Tell him he's the most special 3 yo you know. Tell him you're so glad he's your little boy. Praise him every chance you get.
I wouldn't put him back in diapers. I would tell him that if this continues you will have to put him back in diapers but you know that he's a big boy and can use the potty. Tell him you'll be his cheer leader and then praise him when he does use the potty. Watch for signs that he needs to go just as you did when you were training him and then take him to the bathroom or remind him to go and shout praise as he runs to use the potty.
Focus on every positive thing that he does. Include him in taking care of the baby even if it's to just ask him to come sit by you or come talk with you. He does need extra attention right now.
Pooping and peeing outside the toilet can also be caused by his not paying enough attention and getting there too late. He has a lot on his mind, trying to adjust to this new baby and the way life has changed for everyone.
It can also be a way of expressing anger. When he seems angry, encourage him to express it in words and in action. Give him a place to hit when he's angry. I had my grandchildren pound on the couch. This didn't work for my grandson but helped with my granddaughter. I also play boxed with them which helped with both of them. When they'd say, "I hate you" or start pushing or even slapping at me, I'd say, in a funny voice, "you want to fight with me?" and raise my hands in a fist and start punching the air around them but never actually touching them. After a couple of times they were able to hit the air and not me and we'd end up laughing.
Children often cannot use words to express feelings. So I would express them for them. I'd say something like, "it seems like maybe you're angry or looks like you're having fun." or "if I were you, I'd be mad." Things like that. I learned to not say specifically what they're feeling because even tho I might be right (also could be wrong) no one wants to think that someone else can see "thru them."
Perhaps you and/or your husband could arrange to spend some special time with just the 3 yo. Perhaps he could "help" you fix dinner while your husband watches the other two. Or take just him to get an ice cream cone or to go shopping during which time you also focus specifically on him.
Needing more attention is a legitimate need that needs to be met. Once he feels more secure he'll need less attention. Feeling angry is also normal and at 3 he's still very much unaware of how to acceptably express that feeling. He needs help learning to recognize how he feels and how to express that feeling.
My daughter and now my grandchildren often express their emotions subliminally by drawing pictures or just scribbling. I know my 6yo grandson is frustrated when he scribbles over and over in a circle when I ask him to draw me a picture. This is a clue for me to think about what is happening with him and usually means he needs a quick hug and kind words.
A new baby is a great thing but having one is also quite stressful for everyone. Your toddler may be picking up on that stress and not being aware of what is going on, feels anxious himself. Talking as the day goes on about what is happening and how you're feeling could be helpful.
I remember, before I had a daughter, being at a friends house when her toddler kept whining while she was trying to show me something. She calmly told him that she couldn't deal with his whining while she was showing me something to do with her sewing. She told him to go to his room and she'd come see him when she was finished. He quietly left and when we'd finished she went to his room and spent 5-10 minutes with him while I stayed in the den. Worked wonders for everyone's disposition.
My friend was calm and most importantly, matter of fact. She said how she was feeling and did not accuse him of being whiney. The way in which she did this did not put blame on anyone. It just was what is.
It is possible that there is a medical cause for so much pee and poop outside the toilet. You may want to eliminate that possibility with an appointment with his pediatrician.
Here is my personal experience with this issue. My baby brother was born when I was 2 1/2. I was probably around 3 when I very distinctly remember standing the the hall doorway crying because my parents had put my brother's wet diaper on me because I had peed (in my panties? I don't remember that part.) I remember feeling misunderstood even tho at that age I didn't yet have the concept. I felt alone and scared as my parents lectured me about how I was not to have any more such wetting myself episodes. I can still see the scene in my mind. Years ago I asked my mother about it and she said that it did happen. She apologized and said that she was just so frustrated she didn't know what else to do. She said it didn't help.
I don't remember feeling jealous or upset about the baby. I don't remember anything else about having a baby brother but I still feel sad when I remember this one incident.