Hi
I did my unicef breast feeding course a while (quite a while ago mind!)
Fistly, YOU DID REALLY WELL with all of them!
My ds1 had lactose intolerance too, and the same symptoms, so it wasn't you, it was his allergies, don't feel bad. to go on and feed DS2 for six months was amazing.
You can get a foremilk / hindmilk imbalance, switching baby too often, or if you have a very ample supply, then he will fill up on one breast, then only take fore from the other. the symptom for this IF I REMEMBER RIGHTLY is a gassy stomach and green stools. If you enter 'too much foremilk' into your search engine, there are quite a lot of sites.
Go for it with number three! It can be done- DS1 for me gave up, like yours, ds2 lasted 4 months (nerves mainly), ds3 lasted.... 15 months!!
You've done much better up to now than you think, be proud of yourself girl!