Are Two Hybrids Necessary

I would think through the clubs you have and assess the gaps in distance, if any. Try to fill any gaps over 10 to 15 yards. If you have a gap then pick a hybrid, if that is what you want, that fills that gap. Not knowing what other clubs you have in your bag, and how well you hit your irons, it is hard to tell whether you should or should not buy the 3H. Maybe you could provide the info and we can help.

Why not start with a 4 hybrid?

The longer hybrids can be tougher to hit and the 3 may not be the best to start with.

Agreed, I have a longer hybrid in my bag right now that's very hit and miss. It's a 200 yard club when it's right, it's a 20 yard club when it's wrong and lately it's been wrong more than it's been right. I used to have a set of TM burner hybrids (4-5/21-24) and they were much easier to hit but honestly the gaps weren't enough to keep them vs my 4i so I went to a longer hybrid that is going to take more work. 

As cooke119 you need to fill the gaps in distances. What loft is your lowest Wood? If you have a 5 wood (18 degrees) and start at a 5 Hybrid (about 27 degrees) then a 4 Hybrid (24 degrees) could definitely fill the gap. Maybe even a 3 Hybrid (21 degrees).

Depends whether you are getting good distance on your woods. There's no point packing loads of clubs at the top end if there's no difference in distance.

Personally I have a 24 degree iron, 21 degree hybrid and 18 degree 5 wood.

Depends on your gap between your clubs, and how you want you iron set to look. Would you be put in a situation to use that 3H often, would you be better suited for another wedge rather than a hybrid? really there is no wrong answer, depends on how your game plays. For me, i took my 5w out of my bag, and added another wedge. I hit the 5w maybe 5 times a year. Now, i can always look at a course do some math and figure out if a 5w would fit back in, so you can always get an extra wedge and 3H, and just take the club out and put the other in, depending on the course you play. 

Personally, I like a 5 and 6 hybrid for someone without great confidence in their iron game - 25 and 30 degrees or a similar loft difference and 1 inch between shafts - that assures one of a difference between distance. These clubs can fill a gap similar to a 4 and 5i distance, depending on the hybrid - they vary. Below that a 21-22 fairway works, although I think shaft lengths are too long today - I prefer 40-40.5 in length and with todays adjustable lofts and weight varying clubheads, it is doable. For example, cut down a Titleist 910 7 wood by .5 inches and put the heavier weight in there to offset the difference in swingweight and flex. Adjust the loft up or down to suit your needs, etc.

It's not just about loft gaps.  I have a 24* 4i and a 23* hybrid, but the hybrid flies 10-15 yards further so it's a natural step up.  I replaced both my 2i and 3i with hybrids (18* and 23*, respectively) and love them.  The only thing that really matters is how far and well you hit them, and if they fit your game.

Hybrids are a funny creature - they all differ, shaft lengths differ as do shafts, lofts for the same iron replacement differ, clubhead features differ, launch and spin differ. You've got to test. My preference is to stick with one model of hybrid if I'm using more than just for the sake of some consistency.

Add hybrids as needed.  Really, ask yourself what is the longest iron that you can hit comfortably.  Start there and work your way up replacing the irons with hybrids.  For me, a 5-iron is the longest iron that I feel comfortable with, so I carry a 3H and 4H.