The triptych has been joined and bordered!
After edging each piece with single crochet, I discovered my eyeballing skills are pretty good – the long side stitch counts were all within 2 of each other. I was able to order the panels so that the paired edges were only 1 stitch off, and then put them back to back and joined with a pattern I invented (but am probably not the first to do so): join yarn in rightmost corner of front panel (left if you are lefty) and ch 2; dc dec in same and next st. Then: *2 dc dec in rear panel, 2 dc dec in front panel* across, with no stitches skipped. My long side was 28 stitches, divisible by 4, so I ended with another solo dc dec. To make up the stitch difference, in the middle of the short side I replaced one dc dec with a dc.
I did one round of dc all around the joined panels, which required starting a new orange skein, and then made a back layer for the mat with single crochet. I used up the rest of the green and cream skeins and started a new skein of cream. Both the cream and the orange were nearly all used up by the final border: dc in cream to join the layers, separate lines along the short sides and then the long sides, and then a round of sc in cream; finally, alternating front post and back post double crochet in orange, made around the cream dcs, with slip stitch to round the corners.
It’s pretty cushy.