Of course you can. With an 1818VSL, you can record up to 8 microphones simultaneously. Is your 'audio i/o set up' correct ('Studio One' menu tab > 'Options')? The most straightforward way would be for each input on the 1818VSL (vsl 1, vsl 2, vsl 3, etc.) to be assigned to separate corresponding inputs in Studio One (mic/inst 1, mic/inst 2, mic/inst 3, etc). Then, in your song, make three tracks and set the inputs with the drop down menu to, say, vsl 1, vsl 2 and vsl 3 respectively (or whatever inputs you have your microphones connected to). Pan two of the tracks for the stereo microphone.