how easy is it to change the set up of a t3 from coswort to rs turbo

 

you need a new downpipe to bolt straight onto the exhaust housing, heres a crap picture of mine

 

do you mean you want to fit a t3 from a cosworth to an rs turbo?
if so then you have a few routes to go down.
you can just unboltthe core and attatch it to the escort rs turbo turbo exhaust housing, this will boost more, but will still have the limitation of the 0.36 housing, but you can get a 0.48 housing and retain near the cossie housing potential.
alternatively you can do as pictured above with the complete cosworth turbo unit fitted to the escort rs manifold, just a modified cossie downpipe would be needed.
i did the latter for mine as it flows better than the rs turbo elbow off the turbo unit.

 

i have a stage 3 turbo 48ar housing and 60 trim compressor wheel is this the cosworth turbo got told its the same size compressor wheel

 

yes it is, it's a cosworth sized compressor wheel and housing etc.
basically it's a T3 with a compressor wheel of exducer 60mm and trim of 60, that's what was used on a 4wd cosworth and all the t3 turbines were the same.
a stage3 t3 is basically a 4wd cossie turbo, but has a 360 degree bearing to spread the load of thrusting better.