Mustek is the world's largest scanner manufacturer. Some models have a built in transparency adapter, but I don't know if there are A3 models that have the adapter. The P3600 doesn't, I think.
As you probably realise, to scan slides you need to shine light through them, not bounce light off them, so your HP would need a transparency adapter.
You might want to consider renting a slide scanner with bulk slide feed: doing slides one at a time will be slooooooow.
I use Hamrick's Vuescan, which understands various negative and positive emulsion characteristics, so can do good colour correction/recovery.