First off you may not need to add any yeast at all. How long was it in primary before the 3 month secondary? My last Barleywine was 2 month primary and 4 month secondary and it bottle carbed fine without any additional yeast, it took a month but I had planned on 6 months of bottle aging so no big deal.
If you do add the yeast I'd rehydrate it as per the instructions on the pack, let it proof, give it a stir and then dump it into the bottling bucket racking your beer on top to "swirl" it in, maybe give it a very gentle stir if you want to make absolutely sure it's mixed in well.