Replace column in one file with column from another using awk?

try:

awk 'FNR==NR{a[NR]=$3;next}{$2=a[FNR]}1' f2 f1

Output:

111 000 444
222 111 555
333 555 666

Explanation of the above code:

FNR==NR allows you to work with one entire file at a time. In this case it is the file f2. NR and FNR both contain line numbers with the difference being FNR gets reset to 1 when a new file is read where as NR continues to increment.
While we are working with f2 file, we are creating an array called a using line number (NR) as the key and third column ($3) as the value. next allows us to skip the rest of the action block.
Once f2 file ends, we start to work on f1 file. NR==FNR condition will not become false as FNR will increment from 1 and NR won't. So only second action block {$2=a[FNR]} will be worked upon.
What this block does is it re-assigns second column value to array value by looking up the line number.
1 at the end prints the line. It returns true, and in awk true statements results in printing of the line.
f2 f1 is the order of files defined. Since we want to create an array from file f2 we put that first.

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