I'm a little late to this party, so this is probably something you already figured out a long time ago, but for anyone else who's trying to do this, hopefully this helps.
First of all, it's important to realize that what region of the sequence is considered CDR-H3 can vary slightly depending on what antibody numbering system you use, since each has a slightly different definition of the CDRs. There are (remarkably) at least six different numbering systems in the literature, although IMGT, Chothia and Kabat are probably the most common. This can be a little confusing -- this paper provides a really nice overview of the different systems, the differences between each and how to convert from one to the other. If in doubt, I would just use IMGT. Again, the differences will be slight. But it's still good to be aware that how you define CDR3 will be slightly different if you use say Chothia rather than IMGT.
With that said...MIXCR is a nice program, but it's designed for analysis of sequencing data and is overkill for a situation where you have what are already protein sequences and don't need to do all the preprocessing that raw sequencing data will require. You could use the ANARCI, AbNum or AbRSA online tools, but for a single sequence I would just use the tool on the IMGT website, which will give you both the sequence numbering and some other useful information.
If you have say thousands of protein sequences, you can use the ANARCI tool, but I've found it to be quite slow for any large number of sequences; it does some rather inefficient things like loading all of the sequences from a fasta file into memory all at once, then writing them back to disk to query against profiles for all species and chains in HMMer (even if the species & chain type are already known), then if they are single domain sequences writing them back to disk AGAIN to query HMMer again, then doing lots of corrections to the HMMer alignment, etc. (Shameless self-promotion here -- I've written my own tool which is 50 - 100x faster than ANARCI depending on settings. The strategy I've used is pretty similar to the one in the AbRSA paper, but with some tweaks. The AbRSA tool is also available and is another you could use if you have lots of sequences.)
Once you've numbered your sequence, e.g. using the IMGT website, to identify CDR H3, determine which numbers correspond to CDR H3 and then whichever positions in your numbered sequence correspond to that are the CDR. If you use IMGT, this page will give you the IMGT position numbers corresponding to CDR H3. If by contrast you use Chothia or Kabat, this page has a nice overview of which position numbers correspond to CDR H3 in those schemes.
Long story short: 1) choose a numbering scheme (I would suggest defaulting to IMGT if you're not sure); 2) number your sequence(s) (if you only have one as it sounds like, perhaps just use the IMGT tool); 3) look up the position numbers in the numbering scheme that you're using that correspond to CDR H3 and there you go.
What species are you analyzing? IMGT is a great resource.
Sorry about that the problem was not clearly described. Yes IMGT is useful, but i am looking for some stand-alone tools or correct rules to extract CDR after numbering antibody sequence. I am not sure the definition is enough to do this.