As mentioned in earlier replies, it partly depends on whether you're just signalling to indicate the track / switch settings, or for speed. The farther back in time your layout is set, the more likely the signals would be primarly for the track settings.
I would think you could do it with a '2-over-3' signal. Green-over-red would mean you're going straight on the mainline; Red-over-green would mean you're crossing over to the other main, and Red-over-yellow would mean you're going onto the branchline.
Note that even in a situation where a railroad could get buy with a one-head signal showing just red and green say, they would often add a second "dummy" red signal at an interlocking. Many block signals, which are normally just one head, are 'permissive', allowing trains to stop and proceed or to pass the signal at reduced speed. A red-over-red signal is always an 'absolute' stop, reducing the chance an engineer would misread a one-head signal and derail on a mis-aligned turnout or even run into another train.