use strict;

open my $FILE1, '<', 'my/input/file1' or die "Could not open FILE1: $!";
open my $FILE2, '<', 'my/input/file2' or die "Could not open FILE2: $!";
open my $FILE3, '<', 'my/input/file3' or die "Could not open FILE3: $!";;

open my $FILE4, '>', 'my/output/file' or die "Could not open FILE4: $!";

while (my $file1 = <$FILE1>) {
    my $file2 = <$FILE2>;
    my $file3 = <$FILE3>;
    
    chomp $file1;
    chomp $file2;
    
    print $FILE4, "$file1 $file2 $file3" or die "Could not print to FILE4";
}