Do the best you can, avoid all the obvious non-vegetarian dishes, and don't beat yourself up if you discover you've consumed incidental meat products by mistake (gelatin, rennet, etc). You're just three weeks into this and it takes time to learn all the do's and don'ts. Fill up on side-dishes and have plenty of vegetarian snacks on hand (veggies, cheese snacks if you're lacto, chips, salsa, etc). Most holiday gatherings I've been to in the past have had relish trays and plenty of other veggie items to snack on. Good luck and congrats on becoming vegetarian!