SPCR

Would this be the best cooling combo for Define R6:

• AiO Celsius S36 top - pushing air out

• two front 140mm fans - pushing air in

• one rear 140mm fan -pushing air out

Seems about right - the front intakes can be run pretty slowly, or even be off at times.

What's your CPU? Will there be overclocking/overvolting?

If it's some stock 65-100W-ish CPU, then going with a top mount AIO will certainly be noisier and maybe not that much cooler than a large air-cooler.

Your suggested cooling solution should be fine unless there's a graphics card that also need some air.
With a graphics card the primary concern should be to make sure there's a steady flow of cool air in the lower region of the case.