what?
What do you mean 3 years of the same design? I mean, yea, so? Let me go back a little bit for you, not quite to the beginning but back a ways.
iPhone 4 - squared off edges, glass front/back
iPhone 4s - same as 4, better internals
iPhone 5 - squared off edges like the 4. Minor design change
iPhone 5s - same as 5, better internals
iPhone 6 - rounded edges, all aluminum body
iPhone 6s - same as 6, better internals
iPhone 7 - rounded edges like the 6. Minor design change
.....
Are you seeing the pattern? The iPhone 4, 4s, 5, and 5s were all similar designs. Likewise, one could fully expect the iPhone 6, 6s, 7, and whatever's next to follow suit.
To be distraught about no "major update after THREE years" means you haven't been paying attention.