Ive been thinking about this and just re-read some of the replies.
Some are a bit harsh IMO.
Some people have said [not word for word, but general gist], that they are adults now and must deal with this situation like adults, well, isn't that what they are trying to do?
I can totally understand the OPs feelings of dismay and her not wanting to be catapulted back to the way it is when you have a small baby in the house, I think it is normal to have these feelings. Your teen being part of an unplanned pregnancy is always going to be difficult to process, she also has the added pressure of having to make a decision. Not easy.
It, to me, sounds like they as a couple, are trying to plan a situation where they can both be responsible for the baby/situation that they have created. They want him to move in so he can support her through her pregnancy and so they can both parent the baby, I think that that is actually quite a mature approach.
They have a [possibly presumptuous!] plan of 3 years, but again, planning in that way is mature and sensible, they have the right idea, but are probably going about it in an immature way? stands to reason, they are 18.
Is the 3 year plan so they can save up, get themselves sorted career wise and then set up in their own home?
Most parents don't expect their offspring to leave home at 18, most are open to the idea of them staying at home until a time where they can afford to leave.
Obviously they must pull their weight in the household, the OP says they don't have a great track record of doing this, but show me an 18 year old that doesn't have a similar track record in their recent life history. That doesn't mean they can't change/improve. They must do so.
Yes, they have made things much harder for themselves in getting pregnant at this stage in their lives, that doesn't have to mean they have automatically forfeited any parental support. I don't agree with the idea of them being pushed out into the big wide world and made to 'get on with it', that sounds punative, which is how we deal with children, or criminals. They are neither, they just need support.
OP I have a son similar in age to your daughters boyfriend, he has a girlfriend and if they got pregnant, I think I would feel the same sense of disappointment and worry that you do, but I would also feel proud that he was trying to plan at being a parent and not running away from the situation.
They may not be going about it 100% in the right way, but they are trying.