Correct it where I am Wrong...
S = 3 E = 3 R=1
Strings can be 5 letters or 6 letters or 7 letters..
Seven Letters : (7! / 3!*3! ) = 140
Six Letters : We can take out either S or E or R = ( 6! / 3!*2! ) + ( 6! / 3!*2! ) + (6! / 3!*3!) = 140
Five Letters : We can take out either {SE} or {ER} or {SR} or {EE} or {SS} = ( 5! / 2!*2! ) + ( 5! / 2!*3! ) + ( 5! / 2!*3! ) + 2*( 5! / 3! ) = 90
Final Answer = 140 + 140 + 90 = 370