Hey, Mom, sharing with a three-year-old is always difficult. I have three children (boy, girl, boy) and it seems that when one of them was three the other (or others) were frequently frustrated at having a precious thing destrpyed by the three- year-old. That kid often earned the moniker "Master of Destruction", "Destructonater", "Master of Disaster"...
Here's my advice. Pray. Start slow. Build your five up by explaining three's view of him. Big brother is next to God in three's eyes. Encourage him to see three's desire to do and play everything with big as the hero worship it is. Then try to plan with big a portion of things he'd be willing to play with three, or his toys he'd be willing to play with three and even which toys he might descend from his level to play on little brother's level with every now and then.
Don't forget to balance you older son's concessions with an acknowledement of his need for three-year-old-free play space/time. Three's are destructive, if only because they lack the moter skills older kids have. If three is ALWAYS around, five really can't explore the play/creating activities in which he is intensly interested.
One final piece of advice, pray with him about these issues. Look at it as a way to get some early experience in working to make relationships sucessful. Don't be afraid of holding you older child to a higher, albeit age-appropriate, standard. He will always decry this as unfair. But the truth is, at this age, he is capable of much more self-control than his little brother. And it's going to be that way for several years.
Keep it up. We made it through the preschool years, and now all three of my kids are good friends (ages 8, 11 and 13).