How do I learn 3.5e or 3.P?

[puzzler7]

I know 5e, from reading the basic rules and the PHB.

Is there a single document that I can read to learn 3.5e? What about Pathfinder?

[TotallyNotEvil]

Well, I'd say the 3.5 PHB is a fine place to start.

It will already look familiar due to your knowledge of 5E.

For PF, a quick seach gave me this: http://paizo.com/pathfinderRPG/prd/coreRulebook/gettingStarted.html

[The Vagabond]

To learn Pathfinder, you should most likely read the Pathfinder core rulebook. Fortunately, all Pathfinder content is available on the Archives of Nethys (http://www.archivesofnethys.com/Default.aspx), however, this is all chaotic and mad content, and I wouldn't recommend learning Pathfinder with this material. You should learn through the pathfinder prd (http://paizo.com/pathfinderRPG/prd/coreRulebook/gettingStarted.html), which limits things to the individual books. Read through the section called Core Rulebook, then stray as you desire.

[EldritchWeaver]

3.P is usually called the combination of D&D 3.5 and PF. It depends on the group, if that means that 3.5 is the base and PF stuff is imported on a case by case basis or the other way around.

[puzzler7]

Alright, thanks!

[Psyren]

Your best bet to learn PF, if nobody in your playgroup already knows/is able to teach you, would be the Beginner Box.