Just entering the formula
=RANDBETWEEN(100,300)
in all of the cells should do it. This seems relatively obvious given that your subject line states randbetween.
Every time the workbook recalculates the random numbers will change.
If you want to make them static, copy/paste values over themselves.
Alternatively, if you don't want to (can't) use RANDBETWEEN this formula should also do the same thing.
=INT(RAND()*201+100)