Tagovailoa transferred from Alabama in May.
Jackson is one of four starters from last year's team to opt out.
Williams is the top-rated quarterback in the class of 2021.
The quarterback had announced his intent to leave the program in early February.
Lewandowski racked up 2,804 yards and 28 touchdowns in 10 games for Wisconsin-La Crosse last season.
The Washington Redskins selected Young with the No. 2 pick in the NFL draft.
The Terps are left with Josh Jackson and Lance LeGendre as their lone scholarship quarterbacks.
Pigrome played in 11 games last season, making three starts.
Coach Mike Locksley has focused on building the program's foundation in his first year.
The Terps again couldn't get past their own miscues.