I just integrated my 3 day old chicks with the 10 day old chicks and it was totally fine.
I got 3 new chicks to replace the 3 that didn't make it in the first batch, but I had to wait a week, so I was a bit nervous. I read posts here on the forum that said it would be just fine, but then the girl at the feed store spooked me a bit with her strong recommendation to keep them separated "until they were the same size" (but that won't be until adulthood, right?). So I separated them for about a day, but my instinct was to integrate them sooner than later and here is what I did to ease the integration:
I put both groups (10 day olds and the 3 day olds) into clear plastic bins (shavings on the bottom, so they wouldn't slip) - separating each group, but I put the bins next to each other so they could see one another. Then I rearranged the big brooder that they would all be sharing, added the litter (dry and mostly clean) from the new chicks brooder to the shared brooder, an extra water source and feeder (I'm using nipple water system with 6 nipples and everyone loves it). Once the shared brooder was ready, i put the new chicks in first, let them investigate, eat, drink, find the heat (took about 15 minutes), then I added the older kids (10 day olds) and observed.
It was all fine. No pecking, no aggression - they were all looking around at what seemed to be 'new digs' for everyone. They've been together for 24 hours now and they're great. They all cuddled together last night and some seem to be buddying up. Phew!