Just to be sure: the maximum you can get is CAT IIIa, not IIIb. It is still good enough and you will hardly use it anyways. I used to fly ERJ145 and we had an HGS, too. No auto-throttle, but we were still able to fly manual CAT IIIa approaches. We did not perform many in the end, but the training took a lot of time and once a week we had to fly a simulated CAT IIIa approach to keep up certification/currency requirements.