I have a feeling the answer is going to be no but if I use the 50 watt switch on my Carvin V3 could I use a 50 watt attenuator like this.......I'm just curious because I've been thinking about getting an attenuator for a while but I didnt want to spend $200 or $300.

Personally, don't bother. Waste of $300. It'll still be decent at low volumes.
I think attenuators have to exceed the input theyre given, mainly because valve amps cranked to full usually give out about another 1/4 of there power in wattage and thats probably what youll mainly be doing with it.