Can you combine a play 5 with two stereo play 1's?

Stereo is heard when two speakers are in front, usually at each vertex of a triangle with the listener at the third vertex. An equilateral triangle is ideal, or an approximation of it.

That suggests adding another 5 unit in your case.

It's not rocket science unless you read @Kumar's answer.

Stereo is heard when two speakers are in front, usually at each vertex of a triangle with the listener at the third vertex. An equilateral triangle is ideal, or an approximation of it.

Simply put two speakers (Play 1's etc) make a stereo pair, so you could add another Play 5. Two Play 1's are great as a stereo pair and for the size of my room(s) Play 5's would make the "walls come tumbling down"

I have a 1000 square foot design studio with 11 foot ceilings. We have a play 5 (2nd Gen) at one end (horizontal for stereo) and two play 1's paired for stereo on the opposite end. It sounds great. I have them grouped so I can adjust the volume to both but have the play 5 turned up a bit higher to get the bass out of it. This is a design office but we do crank it up sometimes late night. A sub would have been too much for this space. The bass from the 5 fills in nicely and sounds rich at low to moderate volumes, and plenty powerful at higher volumes. Very happy with the quality and ease of install for the price.

I'd put the Play 5 front center and the left and right Play 1s to the left and right of it, making the Play 5 more or less a center channel speaker.

You could put the Play 5 in the front of the room and the Play 1s at the rear of the room, not sure how well you'll like the sound field from that but for me it isn't bad. I use a pair of Play 3s in front and a single Play 1 to the rear when it hasn't been hauled off to play elsewhere.

I would put the play5 on the ground or near the floor and tune the system so that the play 5 plays more low end to compliment the listening height of the play ones