Are the different LEGO Xtra play mats designed to be used together?
If you mean the different designs (paved road, beach, grassy park) creating a continuous landscape that seamlessly flow into each other, no, that's not the case. However, each of the playmats is double sided with different but compatible designs. So, you could make the paved road, beach, and grassy park longer by purchasing extras of the same landscape and using both sides to create variety. (You would need two 2-packs of the same set to take advantage of all four printed sides.)

While the different landscapes don't fit seamlessly with each other, you could make it work by placing some structure or additional brick-built landscapes at the seams to hide the lack of continuity. For example, between the paved road and the beach, you can place a row of houses on top of the seams to blend the landscapes.