Turn on voltage by using smaller voltage
If I understand you right, you mean something like this:

Where Q1 is a transistor, which opens or closes 5V supply to the load. R1 is 1k-100k (standard is 10k, but it doesn't matter which one, it's a pullup for the gate). Q1 can be a PNP, whose base is similarly pulled up by R1 so that it doesn't conduct in normal condition. If you use NPN or PNP, don't forget base resistors.
ON/OFF is your 3V signal. When it's HIGH (3V), T1 conducts and pulls gate/base of Q1 low, powering the load with juicy 5V. When ON/OFF is LOW (0V), T1 doesn't conduct, Q1's gate or base is pulled up via R1 (as if T1 simply doesn't exist) and the Q1 does NOT conduct.
Hope I guessed your intentions right