Well there was I a iPhone 3G then a 3GS and Then a 4 and a 4s so I'm guessing the next one will be a iPhone 5 not 6
The 3G was named for network technology, not what gen phone it was.
The iPhone 4 is the only one named after its generation. Remember:
1st gen - iPhone (often called iPhone 2G)
2nd gen - iPhone 3G (named so because it has 3G cellular)
3rd gen - iPhone 3GS (the 3G, but more Speedy)
4th gen - iPhone 4
5th gen - iPhone 4S
And I predict the 6th gen will just be called 'iPhone' again.
I see it more like this. Also keep in mind that if Apple names it iPhone 5 (or whatever) that's not a gen marker since no matter how you slice it the iPhone 3 was not a 3ed gen.
gen 1 - Original iPhone
gen 2 - 3
gen 2.5 - 3S
gen 3 - 4
gen 3.5 - 4S
gen 4 - "next iPhone"
There is no such thing as a half generation. Nor is there an "iPhone 3". (And the iPhone 3G was an original iPhone in terms of hardware, with the exception of the 3G radio, so your reasoning fails here.)
Only the iPhone 4 is named after its generation. The rest are arbitrarily named, or named for something else.