If you upgrade lasers,you get a bonux of 6% more damage when you reach max level.In fact,that means for an lf3 you receive : 150(max dmg of an lf3) + 6% = 159 (new damage)
For 31 lf3,you've got : 31*159 = 4929(with upgrade) and 31*150=4650,and the difference are 4929-4650=279.>> 279 / 150 = 1.86.So,finaly you get with 1.86 more laser :)),so,you waste your uridium
For 31 lf4,you've got : bla bla bla bla..:D ...and,after a few math you get with + 2.48 more lf3,or with 1.86 more lf4.
So,in my opinion,don't update your lf3.
For 31 lf3,you've got : 31*159 = 4929(with upgrade) and 31*150=4650,and the difference are 4929-4650=279.>> 279 / 150 = 1.86.So,finaly you get with 1.86 more laser :)),so,you waste your uridium
For 31 lf4,you've got : bla bla bla bla..:D ...and,after a few math you get with + 2.48 more lf3,or with 1.86 more lf4.
So,in my opinion,don't update your lf3.