Just use the C-style for loop. Many people will automatically balk at it, but it's more flexible than the Perlish loop. This is a case where you want to use it:

for (my $i=0; $i<@dynval3; $i+=3) {
    change_value(@dynval3[$i..$i+2])
}
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This assumes @dynval3 has a size that's a multitude of 3.