extra driver or horse
驂 centers on an extra horse or driver in a team, specifically the third horse or an additional horse harnessed alongside a pair.
驂 combines 馬 (horse) with 參, which likely contributes the sound and the idea of joining or adding a third, reflecting the extra horse in a team.
The left side is a horse 馬, and the right side 參 suggests joining or adding. Picture an extra horse being added to a pair, making a team of three.
For サン, imagine the extra horse standing in the sun: sun -> サン, and the third horse waits beside the pair.