I have 3 x business websites with completely different urls, site structures and slightly different content all of which are related to booking live bands.

Site 1: 12 years old, html layout, poor design but still useful for seo, index page PR3, 330 pages, decreasing in organic SEO ranks, 100+ backlinks
Site 2: 18 months old, php, good design and functionality, index page PR2, 6,6600 dynamic pages, increasing in organic SEO ranks, < 10 backlinks
Site 3: Wordpress version of Site 2, labour intensive to maintain, still working on it so PR0, currently 40 pages plus blog posts, 0 backlinks

I would like to use Site 1 and 3 to support Site 2 for SEO purposes. What would be the best way to do this?

I appreciate that many people would suggest putting Site 2 onto the Site 1 domain. However, this would mean losing the SEO rankings / results for Site 1 and having to redirect Site 2 > Site 1 in the future. Is it still worth doing?

I'd really appreciate some advice on what the best way forward is and what the best practices might be to funnel through to Site 2 for improved SEO