I wanted a two year age gap, but for various reasons ended up with an age gap of almost 3 years exactly (just a couple of weeks out). I think it is perfect...
My two DDs are older now (9 and 6) and they are very close. They really are the best of friends, even choosing to play together at school (instead of their own friends sometimes). They have sleepovers in each other's bedrooms and just get on so well, particularly now they go to the park together - it's bliss!
I think the age gap does help to lessen the sibling rivalry, because they both understand that what DD1 does is good for a 9 year old, and what DD2 does is good for a 6 year old and so on.
Both my girls do the same hobbies, and so they go at the same time to do the same things (and I get a few hours peace and quiet). A three year age gap does necessarily not stop them doing them doing the same hobby at the same time ime.
Most of my friends / their friends have similar age gaps too. So there are lots of children in school who have one sibling in DD1s class, and one in DD2s class. That helps, because when do outings out, both tend to have friends to play with
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I agree with the other comments that life is so much easier when they are little too. By the time DD2 was born, DD1 was out of nappies, able to converse and help. She loved being a big sister and would go and fetch things for me to help - nappies, wipes, nappy cream etc... Also, I didn't have to change buggies. DD1 just walked, and when she got tired I put her in the buggy and DD2 into the sling. That made life so much easier. I tried buggy boards, never really got into them. I also just used to take a scooter for DD1, and that worked well too (and bung it back in the pushchair when finished). Its also one heck of a lot cheaper, because the free childcare kicks in, so you're only having to pay for one child at a time.
But, and I say this with a big but... I do think gender comes in to play too. My friend (who has sons) says she could never leave her two together because her DS1 would be too rough with her DS2, and so did not have all the benefits I mentioned. He wasn't interested in being a big brother / helping etc. She puts it down to him being a boy. I wouldn't know.
Also, my friend was desperate to have two close together. She has two years between hers (she wanted closer but it took a while) and hers fight like cat and dog. But, I wonder whether that's because of gender and personality...
In short, I don't think you can say having two closer together in age will make them closer - its down to personality. I know some siblings (same gender) with just over a year's difference who fight like cat and dog. I know another sibling pair (two year age gap) who openly say that they hate each other and do not get on at all, and then I've got a three year gap and have girls who do... In short, I think gender and personality are so much more important as to whether siblings will get on than age (unless there is a big gap of course), that I really wouldn't base my decision on that!