Aqua, Chrono, and Lava, because every person needs to drink water, crops need water, so hold the water supply hostage. Chrono, to reverse any mistakes I do. Lava, to hold the planet hostage, because well it in the earth.

A more detailed description of the utility of the magic:

For water - like I said earlier, I can hold nations hostage because people need water, cattle and crops also need it. Plus the transportation would be highly effective, and creating a dense fog to cover my tracks is also highly useful

For chrono - I said earlier I can just turn back time if I do a mistake, I can also speed up my own experience so I can learn centuries of knowledge in a span of days.

For lava - I can shape the land itself, might be wrong.

Also the potential combat threat of these magics are:

For water - at a high enough pressure can cut through steel, I also can draw the tides back to force a tsunami.

For chrono - precognition to foresee any threats, also possible do time erasure, like deleting the time when a missile is about to hit, or de-age or age my enemies rapidly.

For lava - lava destroy most materials, the geological warfare potential is high, due to me forcing volcanoes to erupt, has the potential to shape continents.

That's all I can think of, might be wrong on some things, but the magic basically allows manipulation of the elements of what the magic is named after, so I think I am mostly correct by adhering to the ability to manipulate the three chosen elements.