Hi schiolier,
3" more between sprockets will need 6" more chain.
At 5/8" per link that makes 9.6, call it 10 extra links.
Stock chain is 104 links for the stock 17T/34T sprockets. Plus 10 more = 114 links.
Most generic chains are packed in 110 links (too short) or 120 links (too long)
If you can't find a 114 link chain you'll need to buy a 120 link chain and crop it.
Do it on the bike, just to be sure.