Can't Get Formula To Work

[rcough]

I'm trying to get a formula to work but really struggling.
What i need is a formula for a cell to auto populate with text based on a letter in another cell.

For Example, if Cell B3 reads Rob then Cell E3 populates with Robert, but if cell B3 reads Sam then E3 populates with Sammy also if B3 reads Will then cell E5 would populate with William.

I have looked online and tried to get the formula to work but keep getting a error on it and its starting to frustrate me, Hopefully this is possible. I'm guessing i may need to change it to a Vlookup, but the cell only has 3 outputs so hoping an IF or IFS formula would work.

Thanks for any help.

 

[MARK858]

Maybe (3 alternatives)

Book1
	B	C	D	E
3	sam			Sammy
4	will			William
5	Rob			Robert
Sheet4

Cell Formulas
Range	Formula
E3	E3	=IFERROR(LOOKUP(B3,{"Rob","Robert";"Sam","Sammy";"Will","William"}),"")
E4	E4	=IFERROR(VLOOKUP(B4,{"Rob","Robert";"Sam","Sammy";"Will","William"},2,1),"")
E5	E5	=SWITCH(B5,"Rob","Robert","Sam","Sammy","Will","William","")