if - else if - else statement and brackets
R reads these commands line by line, so it thinks you're done after executing the expression after the if statement. Remember, you can use if without adding else.
Your third example will work in a function, because the whole function is defined before being executed, so R knows it wasn't done yet (after if() do).
As hrbrmstr has mentioned:
When the initial if is followed by a compound expression (indicated by the {} pair) the parser by default is going to expect the expression followed by else to be compound as well. The only defined use of else is with compound expressions.
In the statement if(cond) cons.expr else alt.expr, the else needs to be put after and in same line with the end `cons.expr' compound.
So if you want to have your code a better look without brackets, apply this way:
if (2==1) print("1") else if (2==2) print("2") else print("3")