Assuming you are serious about downsizing and this isn't just an exercise...
If I were down to just 3 handguns, I'd probably want to stay with one single caliber, three different sizes. My reasoning is one of prepping. Staying with only one caliber means you can sell 100% of your 45acp and 40S&W.
Thus, Glock 19 and the two Sigs fulfill this line of thought.
Plus, since the only reason one would downsize is financial, someone in need of money should probably not even think of owning a Nighthawk.
.45acp, .40 S&W, 9mm, .38 Spl, .380 acp, 22lr
12 gauge, .223/5.56x45mm, 30-30 Winchester
2 Chron. 7:14 Christians must seek His face