Kartik Aryan enters into Hera Pheri 3 and Akshay Kumar won't be doing Hera Pheri 3. Paresh and Sunil will return to reprise their roles, but unfortunately, "Raju" of Hera Pheri won't be returning.

As per reports, Akki had creative differences with the makers regarding his role.
But its pretty much clear its monetary problem. Akki has hiked his rates as well as profit share for every film. Almost all of his latest releases are titled as "co-produced by" Akshay Kumar's production Cape of Good Films.
So its mainly due to % of profit share and base remuneration which caused Akshay's outset from the third installment of Hera Pheri.