So you are using an X7 front mech with an X7 front shifter on a triple chainset?
I think it will be fine. Just get your high and low stops set, and set the middle setting to the middle ring. The two outer rings should then be governed by the stops.
And you suggest a 20 tooth range, and I think MTB mechs can handle 22 teeth. (22-32-44 normally). Sounds like you have one of those CX chainsets.
In my latest experiment I noticed that a Deore 9 speed front mech didn’t get the full triple coverage when used with road STI shifters. A Tiagra road front mech worked fine. I ended up with Tiagra road STI shifter on a Deore MTB front mech, on a road double chainset. Works a treat.