I thought that ALW said that there are going to be three boys playing the role of Oliver and I just presumed it meant that the three he picked would alternate playing the part. But Chester and Gwion(?) have been talking about the semi finals, so is just one of these guys getting the part and the other two actors who haven't been featured on the show?

Yep, I'm hazy about it too, but my understanding is that eight boys will be given their Oliver hats and progress to the semi finals. So we've two Olivers already and over the next six shows we'll get six more. At that point, four Olivers leave the competition. In the final, the three chosen Olivers will be revealed.

I could be wrong, so please correct me if I am. Bit of an unnecessarily drawn out process really.

I assumed another 4 will get to the semi-final.
Then of the 6 the last 3 would be chosen but that doesn't make a semi-final and a final does it?

Ahh ok. For some reason I thought next week was the last Oliver they'll pick

I assumed they'd have 6 in the semi final (since there was 12 in the live shows, a quater final) - then they'd cut that in half for the semi to get 6 semi-finalists, then cut that in half into 3 finalists/winners who are revealed in the final?

Four get chosen; three as Oliver and one as a reserve.

But I think the other 8 are offered parts in Fagin's gang so undoubtedly will play the role of Oliver somewhere down the line.

I'm sure that's right, it was mentioned at the start of the series on here.