Hugs to you. We had a terrible napper and it's really tough. I can't imagine how tough with a active toddler around too! It took a long time with my 7 month old but FINALLY she naps pretty well.
A couple of things worked for us: I read Healthy Sleep Habits, Happy Child. I loved this book because it was written by a pediatrician and just had what I found to be sound, sensible advice. We did have to let her cry, and as gut wrenching as it was, it was much worse to have her exhausted and unhappy because she wasn't getting enough rest. But I know every mom/parent is different. You know your baby best and if that won't work, don't let anyone tell you that that's the only way. I will say it took us about a month to really get it down to where she takes 2 decent naps almost every day.
As for tricks, we established a really regimented routine before her naps that doesn't ever vary. I think that's really what helped the most. It always begins with a 5-10 minute wind down (either a DVD, a book, maybe a bottle or nursing if you're BF'ing) and then I put her down the same way every time (same song, same "it's time for a nap" etc). I'm sure that's tough with a 3 year old but maybe you could all read a book together and then maybe even have the 3 yo "help" put the baby down?
Also, we use a sleep sack for naps so that she knows now when that goes on, it's nap time. (We don't use one at night though.) I think the routine and doing things at nap time, just helps them learn that that's what's going on. Most babies are such creatures of habit and thrive with repetition and routine. But again, they're all different. :) Good luck!!!!