Having a stats problem; wondered if anyone could help

I am doing a meta analysis of various studies. I want to look for differences in hazards between treatment A, and controls B and C.

The problem is that all the studies (except my personal data) look at the analysis in a different way from this. They report hazards ratios for A versus B, and B versus C.

This is not what I want. I want to find a way of collapsing B and C into a single category and using a cox proportional hazards model comparing A against B AND C.

Could anyone advise me whether it is possible for me to take any of the associated statistics and back-calculate a single hazard ratio and standard errors for this hazard ratio? Note that I won't be able to access any raw data for the other studies.

I see that it's not possible from using hazard ratios alone, but perhaps with coefficients?

I have been trying to assess the literature, but haven't found anything of use so far.

Any help you may have would be greatly appreciated!

Thanks!

Oliver