(Disclosure: This site contains product affiliate links. We may receive a commission if you make a purchase after clicking on one of these links. Read our affiliate disclosure for more info.)
GOG has a promo running for the Trine Trilogy.
If you own the first 2 games you can preorder Trine 3 for 25% off. Otherwise you get the standard 15% off.
However the first two games are on sale for 75% off until July 1st.
Trine 2 Complete Story $5
Trine Enhanced Edition $3.74
Trine 3 $18.69 (15% off)