Encroachment of land

Here is the complete story: Around 2002 I (say X) bought a piece of land. It was registered to me by the owner but there was no name transfer at that point. After say some four years(2006), some of the adjacent land (say the land on the left side of my plot) along with my piece of land (my land + land on my left) as a whole was sold to a second person(say Y). The other adjacent land (say the land on the right side of my plot) belongs to a third person (say Z) and let us say he bought this land around the same time I bought mine, that is 2002 but his land is not part of what was sold to person Y. Now around 2008 the person Z built a house on his land and also covered some adjoining area of my land. In next couple of years they extended their house and covered all of our land. How should I deal with this now ? Thanks a lot in advance. P.S - I would appreciate every little piece of genuine advice and support. The excuse for not taking an action earlier would be bad finance and it's actually my dad who bought the land.