it is not so straight forward,
it has to depends on what format you use, 3/4, DX or FX, the larger the format, the narrower the depth of field, as in cover the same area on different format.
it also have to depends on what lens you use, if it is a wider focal length, you have deeper depth of field,
and don't forget how much areas you need to include, if shooting three persons in full length, you can get away with a larger aperture...
than, you need to depend on the lighting condition, see whether how much can you compromise with the ISO, shutter speed and aperture.
so, why ask for a f-stop and where to focus when we have little information on all the things except knowing need to shoot 3 persons in a row?