Can I output video single wire RCA signal out of 3 composite ports?
There are two common video connection types that use RCA-style plugs:

Composite video uses a single connector, typically yellow, to send the video signal. This is really the lowest quality video connector available, capable of sending only about 480i (which is half the resolution of a DVD, and downright pathetic compared to the requirements of high-def video).

Component video uses 3 connectors, typically red, blue, and green, to send the video signal. Component video is analog (unlike HDMI, which is digital), but is capable of sending full 1080p high-def video.

Neither of these standards includes audio, (like HDMI does). Analog audio RCA connectors are typically red and white (or sometimes red and black), and are in addition to the video connectors.

Unless you have absolutely no option, I would strongly discourage you from using composite video. The quality is quite poor by modern video standards, although of course you also need to consider the resolution of the projector. Does your projector support component video? If so, there are HDMI-to-component adapters available (I have no experience with them) that you could use from the Roku if you need to be able to terminate the cables yourself.