I agree, it's not the best RF. The signal varies so much as you walk a few metres inside and outside, like it's oscillating with position, which implies interference patterns.

Absorption will certainly make that worse, but it's also an issue of band diversity: More diversity (even using the same aggregate bandwidth) would tend smooth out the peaks and troughs in reception. If the operators collaborated better they could have the same slice of the pie each as they do now, but make the reception more consistent for everyone.

Still, none of this explains why 3 sometimes shows great reception but little or no data throughput!

Another problem with 3, at the moment, is they really don't like you using 2G mode, because that uses their partner networks and costs them. So all the 3 phones I've used to date will lock onto a very weak 3G signal rather than a great 2G signal - even just for calls and texts. The result is poor reception in those circumstances.

That's something which might conceivably get better with the T-Mobile infrastructure merge - because T-Mobile have 2G hardware themselves, and 3 clearly don't.