I have an issue when it comes to buying the right item against a certain enemy. Like for example, I have a very hard time against 
does anyone know what items am I supposed to start with against these three? And what should Trundle focus on building?
against teemo and vlad, 
against nasus,
Vlad Teemo = Visage Ravenous Nasus = Any Hydra Tabis Bramble and if ur having trouble vs nasus's W i actually recommend phase rush even though its not a good keystone atm but its very good vs his W You can proc it with ur pillar then auto Q to run away.
There are quite a few ways you can build him, but right now I would highly suggest prioritizing hp and resistances.
Against an AP you could go something like this 
and build some damage later. Then you will be more useful to your team during the mid game. Unlike most bruisers, Trundle has enough utility to stay relevant without building damage, so just build to stay alive against a lopsided matchup.
