you won't need 3 engineers until SS5. 1 Engineer and 2 reps will get you through SS4 unless you choose to use gunners on the SS4 instead of reps for the flak guns

best choice would be to start a KM bb crew so u have enough supportcrew for any sub (remember the sprite SN ss6 has already leacked and subs will get a hugh rebalance).

If u dont want to start a bb crew i really recommend u to get 2 more engies for ss5. Its really worth it. but as already said, if u just want to go for ss4 u wil be fine with 1 eng and 2 reps altough i recommend 2 gunners since at least i had a lot of fun cutting through cvs with them.