Hi Ish,
Signal it as you would any other interlocking - a 2- or 3- head signal ahead of the interlocking controlling the diverging routes. Use a 2-head if medium speed is sufficient, or a 3-head if slow speed movement is needed. The signal should show "normal route" or straight-ahead aspects if the switches are line for the mainline, and some sort of diverging aspect if the switches are lined for one of the other routes.
Going the other way, a.k.a. trailing-point on the "main line", if there is one, would use a 1-head signal that shows normal aspects since going straight through is the only path option. The other two lines, the line ends that merge into the single-track, will typically get 2-head signals that show some type of diverging aspect e.g. red in the top head and yellow or green in the lower head. Again, these would be 3-headed signals only if slow speed were required. Don't assume that just because you're underground that speeds need to be slow.
(And if we're talking signalling on Marsz, maybe your "subway" operates at maglev speeds! lol)