They appear to all be 3.5 Player's Handbooks. However, they have different covers, etc.
The first link is the Player's Handbook 2, which adds many rules, feats, and classes into the game. The second and third are the Player's Handbook, which has the core basic rules needed to play. The difference between those two is whether they have extras in the back.
Player’s Handbook and Player’s Handbook II are different books, with basically no overlapping content. Player’s Handbook II is basically a “sequel” to the Player’s Handbook. The differences between the Player’s Handbook and Premium Players’s Handbook are slight, however.
If you are looking at buying some of these books and getting into the game, do note that almost all of the material from five books:
- Player’s Handbook (basic rules for creating player characters and how to play them)
- Dungeon Master’s Guide (basic rules for running the game, playing monsters and non-player characters, developing campaigns, and so on)
- Monster Manual I (stats for a variety of monsters), Deities & Demigods (rules for gods and the like, rules for various alternate religions, particularly from real-world myths)
- Epic Level Handbook (rules for playing past level 20; they don’t really work very well though)
- Expanded Psionics Handbook (rules for psychic powers rather than spells; pretty high quality stuff)
- Unearthed Arcana (variants, optional rules, and so on)
have been released, by Wizards, for free under the Open Gaming License (OGL). The various OGL materials were then compiled into a System Reference Document (SRD), which was then put on quite a few different websites. My preference is for d20srd.org.
Also, all errata is and always will be free, and most SRD websites (including d20srd) simply incorporate errata into the site.
There are some notable missing features from the SRD. Most importantly, all description of when you level up, and exactly what steps you take when you do so, are not Open Gaming Content and won’t be found in the SRD. Paying careful attention to the rules that state what you’re supposed to have at each level covers the how’s, though it’s not all in one neat place. The exact amounts of XP that you need to level up, and the ways of calculating how much XP a given encounter is worth, are also missing. If you like, it’s entirely possible to use “narrative leveling” – having the characters level up when it is thematically appropriate to do so.
On the Dungeon Master’s side, monsters identified as “Product Identity” (i.e. monsters unique and important to the Dungeons & Dragons brand, like Beholders and Illithids) are not included in with the rest of them. There aren’t very many of these, though.