3 Way SLI with two gtx 760 and one gtx 750 ti

Hi i have built a small workstation with 2 gtx 760 4 Giga GPUs, intel core i7, 16 Giga RAM

i want to increase my GPUs but i don’t have money to purchase another gtx 760 and i rod that SLI require identical GPUs in model and in memory size but someone on The internet write something like extending SLI with different GPU but it will not be part of SLI
i want to know about that and if i have to purchase another gtx 760 will i find one gtx 760 after 6 months from now ?