Here’s one method …
hcitcl> set in 1234567890
hcitcl> regexp {(d{3})(d{3})(d{4})} $in matchAll matchOne matchTwo matchThree
1
hcitcl> set new “$matchOne $matchTwo $matchThree”
hcitcl>echo $new
123 456 7890
And here’s another …
hcitcl>regsub {(d{3})(d{3})(d{4})} $in {1 2 3} new
1
hcitcl>echo $new
123 456 7890