I've not had time to read most of the other posts (but this is MN so I can hazard a damn good guess at some of the content )...at the risk of probably bucking the trend I would say:
- You've been together long enough to have 3 kids and are (presumably) fairly happy in the relationship. So it obviously hasn't been affected by his drug use until now. In fact it sounds as though he has 'come clean' (if you'll pardon the expression) entirely of his own volition and not due to an 'ishoo' such as you finding drugs in his sock drawer....
- He's been (again, I am assuming this because you are still together, 3 kids in) a fairly good father to his kids
- He has successfully hidden this from you for all the time you have been together, so it clearly can't be impacting him that much.
So for all those reasons, probably not a big deal, and (IMO) not something worth breaking a relationship over.
However (and here, I apologise for becoming a bit suspicious and simplistic) the fact that he has now told you causes me some concern.
For example, now that he is 'out' is he planning on opting out of Sunday morning childcare due to a come-down where previously he would have made an effort (or taken less drugs the night before and therefore be feeling less rough?)?
By telling you, is he going to treat your knowledge as carte-blanche/approval? In which case you can certainly make it clear that you don't approve/like what he is doing but don't regard it as your job to tell him what he can/can't do (there's obviously a difference)
Is he going to use your knowledge as an excuse to bring drugs into the house? Clearly an sbsolute no-no.
TBH, its down to you to figure out how you feel and set the ground rules (e.g. no driving the kids anywhere for 12 hours after drug taking, or whatever else you feel is appropriate) and also to make sure you don't end up feeling 'used'. If your dp ends up getting a 24 hour exemption from childcare on a monthly basis as a result of his binge with the boys, perhaps you should be entitled to a similar 'break' on a different weekend? Or somesuch. IMO, you don't want to end up being the only adult in the relationship, or you will start feeling very frustrated, so make sure he is taking responsibility for the outcomes of his choices and his behaviour.
Having said all of that, back to where I started: you've been together this long and had 3 children and it hasn't been an issue, so its entirely down to you whether you want to make it one now.
Good luck