As you show, a Y joining two ESCs, than another Y joining those two to the third ESC ... then feeding the last single all-combining lead to the Rx, would do it. You would need the sever the red power leads from two of the three ESCs.... leaving just one BEC to supply power to the RX.

You might be better off with a UBEC (e.g. Castle 10A) and not use the integral BECs in the ESCs at all.