Arturas:
If players 1, 2 & 3 go off for tactical reasons to be replaced by 19, 20 & 21, then if there is an injury to 19, 20 or 21 then that player can be replaced by 1, 2 or 3. If all of 19, 20 & 21 get injured, then 1, 2 & 3 can all come back on.
However if 1, 2 or 3 were replaced due to injury, then they are NOT allowed to return to the field of play in the event of an injury to 19, 20 or 21.
All this is provided the local regulation would allow there to be the necessary number of replacements to do this, and that the team had not used up it's allocation of allowable replacements.