In python "else if" is spelled "elif".
Also, you need a colon after the elif and the else.

Simple answer to a simple question. I had the same problem, when I first started (in the last couple of weeks).

So your code should read:

def function(a):
    if a == '1':
        print('1a')
    elif a == '2':
        print('2a')
    else:
        print('3a')

function(input('input:'))

No, it's not possible (at least not with arbitrary statements), nor is it desirable. Fitting everything on one line would most likely violate PEP-8 where it is mandated that lines should not exceed 80 characters in length.

It's also against the Zen of Python: "Readability counts". (Type import this at the Python prompt to read the whole thing).

You can use a ternary expression in Python, but only for expressions, not for statements:

>>> a = "Hello" if foo() else "Goodbye"

Edit:

Your revised question now shows that the three statements are identical except for the value being assigned. In that case, a chained ternary operator does work, but I still think that it's less readable:

>>> i = 100
>>> x = 2 if i>100 else 1 if i<100 else 0
>>> x
0
>>> i = 101
>>> x = 2 if i>100 else 1 if i<100 else 0
>>> x
2
>>> i = 99
>>> x = 2 if i>100 else 1 if i<100 else 0
>>> x
1