Ascraeus Mons (top right), Pavonis Mons (middle) and Arsia Mons (bottom) are aligned on the Tharsis bulge . Olympus Mons can be seen in the upper left hand corner. The three volcanoes are from 350 to 450 kilometers in horizontal extent and each rising about 15 kilometers above the surrounding plains (or 26 km above the Martian datum).
Source: NASA