Do all Iphones take the same cases?

I'm getting an Iphone 3 and I wanted to know if cases for Iphone 2 3 4 5 all work? They look the same, but I just wanted to make sure

No…

2G, 3G, 3GS, 4, 4s, and 5 all have different dimensions and take different cases for each

No. The iPhone 3G and 3GS are a different shape than the iPhone 4, 4S, and 5. The iPhone 4 and iPhone 4S can fit in the same case, and the iPhone 5 needs a different case. It is because each iteration of the iPhone has a new design and a new shape/thickness. Just make sure when you buy a case, it says "Works with iPhone 3GS" or other phones. If it isn't your phone model, it won't fit.

There's no iPhone 2 or 3. Its 2G, 3G, 3GS, 4, 4S & 5.

To answer, no they can't. The 2G to the 3G/3GS was dimensionally different as was the 3G/3GS to the 4/4S* & the 4/4S to the 5**.

*the 4 & 4S had 2 different models with slightly different positioned headphone ports, silent switches etc
**the 5 is the most dramatic change as it no longer uses the 30-pin connector at the bottom, the headphone port is also at the bottom & the display is 4" not 3.5".