1-2-3

primo = firstly
secundo (pronounced se - gon - do) = secondly
tertio = thirdly (pronounced ter-see-yo)

You can also use deuzio (also spelled deusio) but it is less formal (although it is listed in my dictionary). If you want to be facetious, you can also try uno-deuxio-troisio (simply appending -io to the numbers) but that's definitely incorrect. And lastly, you can go a little bit further (quarto = fourthly) but it becomes a mine field trying to go past 4. I wouldn't recommend it :)