D.B.
Truthfully, I don't think 3 year olds understand themes! I think parents go to a whole lot of trouble for stuff the kids don't appreciate, and other than a few staged photographs, you don't have much to show for all your trouble and expense! If things get too large (20-30 guests), then the kids get over-stimulated and it turns into a free-for-all.
We always kept our parties very small. Just family until age 4 or 5, then the "1 guest per year" rule. When they turn 5, they invite 5 friends, at 6 they invite 6 friends. With twins, you might increase this a little, but I wouldn't double it. A small group lets them make time for their real friends, and lets them open a few gifts in front of the people who give them. My son always absolutely hated the 20+ kids at a party - he dropped his gift at the gift table and never got to experience the recipient opening it and saying "thank you." Nobody learns any party "etiquette" (like how to be a good giver and a gracious recipient, even at the young child level. Then the birthday kids have no energy to write 20 thank you notes and the whole neighborhood/preschool/soccer team winds up in a year-long gift trade with a party every other weekend! It gets overwhelming so it's best to not set up ridiculous expectations you'll have trouble managing when they're older and have separate sets of friends.
For age 3, I'd have one simple craft, and you can get great ideas at your local craft store (Michael's or whatever you have in your area). Things like gluing foam shapes to a picture frame (usually also foam) are easy for little hands, and then you can email a photo of the child to the parents after the party. The craft becomes the party favor. Most parents don't like all the cheap party favors that break. You could do something like finger painting if you can do the party outside and have enough smocks and adult help to keep kids from ruining their clothes. If you cut arm/neck holes in some old pillow cases, that works great. If you have a sprinkler or kidding pool, have the kids wear bathing suits and just rinse off after the activity. 3 year olds don't follow game rules very well, so I'd skip that.
We used to buy those plastic Easter eggs and put a few candies or plastic dinosaurs inside, then hide them around the yard (in the flower pot, the downspout, the lowest notch of a tree, etc.) and have each adult help a child find things in his/her color (so everyone gets the same number of eggs). The contents are taken home instead of a standard goodie bag.
Keep the party short (90 minutes max) and keep in mind that kids this age often still nap, so avoid the hours when they're likely to be melting down.
I think cupcakes work better than a sheet cake. You can get a couple of flavors and the kids can choose what they like. Simple decorations - a few balloons or crepe paper streamers, and you're done! Don't clean out your bank account for these things - kids don't notice and parents don't need the stress.
Have fun - that's the point!