Exactly, the biggest drawn one would likely be the sun in the foreground, the front side of the planet, and the smaller one's would be the suns further back. This is also the safest assumption too, instead of assuming the bigger suns actually just giga huge suns and in the back and a super tiny sun is the foreground one.
This is also why all 3 must be on panel, if only 1-2 drawn, we don't know if either in the foreground, they could both be behind the planet, but if all 3 are drawn, one must be in front.