The index is evaluated first. See JLS section 15.26.1, in particular:
15.26.1. Simple Assignment Operator =
...
If the left-hand operand is an array access expression (§15.13),
possibly enclosed in one or more pairs of parentheses, then:
First, the array reference subexpression of the left-hand operand
array access expression is evaluated. If this evaluation completes
abruptly, then the assignment expression completes abruptly for the
same reason; the index subexpression (of the left-hand operand array
access expression) and the right-hand operand are not evaluated and no
assignment occurs.
Otherwise, the index subexpression of the left-hand operand array
access expression is evaluated. If this evaluation completes abruptly,
then the assignment expression completes abruptly for the same reason
and the right-hand operand is not evaluated and no assignment occurs.
Otherwise, the right-hand operand is evaluated. If this evaluation
completes abruptly, then the assignment expression completes abruptly
for the same reason and no assignment occurs.
TL;DR: The order is 1[2]=3
