Can anyone provide a website which has a downloadable PDF version of a 2000 Triumph Adventurer Service Manual?

http://zkmc.org/pdf/1014488-Triumph-TT-600-03.pdf

Like this:

http://search.yahoo.com/search;_ylt=Ai_SPb4UcyOCCrA0IgGxHASbvZx4?fr=yfp-t-122-s&toggle=1&cop=mss&ei=UTF-8&p=2000%20triumph%20adventurer%20specs

Consult your owners manual, but if you were not provided with one try:
http://triumph.co.uk/usa/5797.aspx

where you may download an electronic version (PDF) depending on model.

is it three cylinder

Most oils are preferential, but don't use car or low friction special oils in your bike, and beware of detergent oils if you don't know the service history, they will break down in a bike and can dislodge accumulated crud. To be safe10/40- 20/50 Motorcycle oil for engine/ part synthetic if you like, but not a necessity sae 80s gearbox, Dot 4 clutch and prepacked branded long life coolant with integral lubricant for cooling.