Building an elevated deck over 6x6x6 posts, a beam made of 3 pressure treated 2x8x16' is too narrow
Getting the right size bracket isn't a problem, if you go to Simpson Strong Tie web site, or other comparable brand, you'll find a part number to fit your beam and post sizes.  Look at the PCZ6.  Or on this catalog page, the CC4.62-5.50:

http://embed.widencdn.net/pdf/plus/ssttoolbox/ef6qdoh2bf/C-C-2019-p090-091.pdf

Just make sure that the built up beam you made is adequately sized for the load.  A built up  beam made from three 2x8's may be strong enough for your deck but it is not as strong as a 6x8 beam.