texture sample float3?

I have a post process material in which I am adding together several SceneTextures with modified UVs to do a blur effect, I would like to mask out some of the the texture before the blur, but I can’t see a way to modify the UVs after I have done this

I could apply the mask as a separate post process material and put that in the blendables before the blur, but then the mask becomes permanent, so I can’t get an unmasked scene texture to blend back on top any more

I thought I was close using texture sample in a function to expose the texture input, but unfortunately this input will only accept t2d format

is it at all possible to sample from a float3 source, or to convert float3 to t2d?