Between getting Trinity and F-Fist, get Trinity always. Your Q already does what F-Fist's passive includes, and Trinity scales much better into late game. Especially in a meta where ArPen has gotten very strong.
If Nashors and Wits End were released in their current PBE status, I would definitely consider picking up Wits End against a heavy AP comp. Nashor's, if you build him AP, I could definitely see this being a strong choice.
Trinity is really only good on Skarner in 3s. The only thing Trinity Force does for Skarner is just make you better at 1v1ing people. On Summoner's Rift where Skarner's ability to initiate and peel for teams is too valuable to ignore, Trinity is mostly pointless, and buying it is basically spending nearly 4k on an item that still leaves you really squishy.
The best offensive items on Skarner are ones that not only give him damage, but also amplify his survivability. Hence why I recommend Gunblade, Wits End, and Botrk. Gunblade and Botrk are furthermore usually better than Trinity for guaranteeing Skarner can land a successful ultimate. IG is better than Trinity on Skarner, but situational and competes with Frozen Heart.
Nashor's is almost good on Skarner except for the fact he's melee and the item provides almost no survivability stats, making it hard to negotiate an item slot for.