[R-meta] aggregate data continues outcomes

Homa,

I assume that you want to compare 'B or C' vs 'A', the following article 
describes approaches to aggregate data from a multi-arm study (you find 
a worked example in section 4.1):

R?cker G, Cates CJ, Schwarzer G. Methods for including information from 
multi-arm trials in pairwise meta-analysis. Res Synth Methods. 
2017;8(4):392?403.

Best wishes
Guido