Basement bathroom setup/drain

I am in process of planning a new bathroom (sink, toilet, stand up shower) in my unfinished basement (soon to be finished). The house is relatively new and the basement was planned for a bathroom as there are 2 drains located close together. One is flush with the floor with a plug in it. I am pretty sure that is for the toilet and all I need to do is put a inside fitting flange on it. The other is a 12x12 framed in hole in the cement floor with a horizontal pipe sticking halfway into the framed box. I am assuming that is for the shower and I have to fit my own p-trap for that. My question is where should the sink drain too? Does the sink drain share the shower drain and is my setup normal? I am thinking the shower pan is supposed to go right over the 12x12 boxed drain after being filled with mortar...but don't get how to do the sink drain. I figure there is a common answer since there is even a vent pipe brought down to this area already.
thanks in advance for your help.
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