I think all of the active Mars-orbiting spacecraft can relay data from landers. The protocol is called Proximity-1. It operates on UHF frequencies in the 400 MHz range (one of them, 437.1 MHz, is in a ham band but because it's at another planet I won't complain). When the orbiter comes up over the horizon at a lander, it sends an interrogation. If the lander has data, it responds and transfers the data up to the orbiter. The orbiter then stores it and relays it to earth at the next opportunity.
This works very well. The usual rule for Earth/space communications is to put as much on the ground and as little in space as possible, but that rule is reversed at another planet like Mars. Because it's so difficult to soft-land on the surface, and because of the relatively long and cold night, you want to keep as many elements in orbit as possible, especially large and heavy items like batteries, high power RF amplifiers, solar arrays, and large steerable communication antennas. That makes an orbiter better able than a lander to achieve a high data rate back to earth. Because of the short range to the satellites above, the landers can transmit at a high data rate with minimal power and a simple, nearly omnidirectional UHF antenna that doesn't need any pointing. The high speed overcomes the shortness of the passes made by the orbiters over the landers. Relaying also allows data to be relayed when a lander is on the night side of Mars and out of direct communication with earth.
