Question Timing for connecting 21, B2 2 F3 (old BnB)

[Train]

ok guys I started to play Flash few days ago , and I have trouble with his old Bnb (112 xx [LK](mb) 21, b22f3 xx Lightning Charge). Simply, I drop that 21, b22f3. I spent a lot of time training that combo ,and no progress.

Do I need to wait for a target to start falling down (after LK) so I can connect 21 (it's 3 punch string), or I need to go very fast? (if I go fast, third punch from 21 will miss,and than I can't connect b22F3)
If i wait a bit for 21 to connect all 3 hits, I miss b22f3 cuz target is already on the ground.

Please some help with timing

 

[FrozenG3oX]

just do 11 b22f3, everyone uses this one cuz you won't drop it if you're near the corner

 

[PPJ]

step forward for a very slight moment then do the 2,1