You will need the d20 Players Handbook (3rd Ed) to play Slaine or you could use the Mongoose Pocket Players Handbook. Though all you really need are the feats and combat section. Its more a case of needing access to the information, rather than needing to own it.
The information not covered are attributes and character generation, combat, skills and feats. If any one in your group has a copy you should be fine.
Alternatively you could possibly keep an eye on Ebay for a 3rd Edition Players Handbook and get one on the cheap.