Of right after posting I find the solution…
It´s so weird that I can never figure it out, looking into it for hours at times but as soon as I make a post I almost always find a solution.
So for anyone having a similar problem:
float3x3 TestMatrix = … still doesn´t seem to work but matrix<float, 3, 3> does!
It was giving me an error because I forgot a silly semicolon before defining another matrix and because the error didn´t return any line number I thought the error affected every matrix (like it did with float3x3) and not just one of them.