The only way I could see that as being beneficial is if you hit them different distances. Or, if they produce different ball flights, ex. one lands softer, one rolls out better, etc, etc. I personally like to have different lofts that give me 10-15 yards difference per club.

I reckon you'd be better off with an 18 or 19 degree 3-wood if you're going to go with a 21 degree 4-hybrid, but it really depends on your club distances and dispersion patterns. What is the difference in total carry yardages for the two clubs? This might help to narrow your choices down to make the best decision. Keep in mind that you should have a gap of about 10-15 yards in between clubs. Hope this helps.

They don't have a 21 degree 3 wood. Do you mean a 7 wood?
Either way it's usually a personal preference for one or the other. Not both, unless you plan on swapping them out for different occasions.

21 degrees in a fairway wood would be a 7 wood, not 3 wood. Both have their uses, but you are not likely to put them in the bag at the same time. One would replace the other given course conditions you would be playing on that given day.

While both are 21 degrees, they are not fully interchangeable.
The obvious difference is the standard length of the 7 wood shaft is longer than the hybrid. A generalization is it will be longer.
If you tend to drive in the fairway, the 7W maybe preferable. With a smaller head, the No de plum is a rescue club the hybrid make work better than a fairway from tough lies.
Work with a fitter that fits your need - not every golfer is comfortable hitting both.

I feel this is one that I struggled with for awhile, but not in the same way. I recently went through a love hate with hybrids, 17 and 21 degrees (4 iron replacement). I then added a 25 degree to replace my 5 iron. I ended up with the 4 iron and the 5 iron back in the bag after struggling with the ball going left when I would get a little aggressive. I went back to the 4 and 5 iron for a few months. I have been playing a TSR3 10 degree as a driver and love it. That made me want to revamp the entire top end of my bag, as I don't need a three wood of the deck often, instead I like it off the tee as a driver replacement. I pulled the trigger on a TSR2+ 13 degree, a TSR3 16.5 degree and a 21 degree TSR2, my 5 iron fills out the top at 25 degrees. I pretty much use the TSR2+ when I need to lace something dead straight or get uncomfortable off the tee. It settles me down nicely when I get sloppy off the tee. The TSR3 16.5 is the perfect combination of height and accuracy from 220-240 from the fairway. The 21 degree just launches high and lands softly from the deck and from the rough. It's also been reliably straight where my 21 degree hybrid liked to move right to left too much for my liking. It's a smooth swing for 200-210 from the deck and can get it out nicely from a peg off the tee when 220ish all in is needed for a fairway or par 3. Your mileage may very, but I am digging the 21 degree TSR2 over my former hybrid.
Cheers!
CJA