That's not obvious to me, but in fact, http://handbook.cochrane.org/chapter...ing_groups.htm says just that:

If there are more than two groups to combine, the simplest strategy is to apply the above formula sequentially (i.e. combine group 1 and group 2 to create group ‘1+2’, then combine group ‘1+2’ and group 3 to create group ‘1+2+3’, and so on).