what one of the three?

[The deerslayer]

I know someone who is buying a gun for protection, but is having trouble on making the decision on witch caliber to get. It has to be a 40s&w, a 45 auto, or a 357 mag. What one would be most likely to give the best performance?

[Lawdog]

The deerslayer,

Lots of variables here.  How experience is the person you are talking about with handguns?  Is this personal protection or for around the house?  Me, for my personal carry, I pack a .45 auto.  In fact all my guards now carry the .45 for a duty weapon.  The .357 in a revolver is also very good and I believe, somewhat easier to master than an auto.  Lawdog
 

[Savage]

Not enough information to make the call. Too many variables, including my own personal preferences. Any of the above will do the job.
Savage

[The deerslayer]

The person is an in between pistol shooter not an expert or beginner. The gun would be for home and carry.

[Savage]

Well, here come the personal preferences. A .45 compact of some sort. The one's I own and like the most are the Glock mdl 36 & the "Officer's" 1911. Big enough for a house/range gun, & small enough to carry under most circumstances.
Savage

[greenjeans]

These are all just my opinions and preference. Springfield Ultra Compact in .45cal has become my primary carry weapon recently. Small enough to conceal, but heavy enough to shoot without hurting. Comfortable enough to punch paper with. Had a Glock 27, .40 cal, and it shot really well, but did not like the width. Can't beat good .357, but again, too wide for me. Smaller concealable ones hard to shoot, especially for a beginner. Think the Ultra Compact is going to work out well.