The passive- aggressive foreign policy approach could well prove to be a failure and there doesn’t seem to be any larger strategy behind the three moves , but it’s really hard to argue they are errors. It is more accurate to regard them as reversals of previous errors.
Canceling the Iran deal is no mistake. The deal foolishly removed pressure from a hostile regime. It convinced the ayatollahs in charge they could disrespect the US with impunity. Obama and his crew signaled weakness and ended up having to deliver pallets of cash as ransom for our sailors. Trump understands the transactional dynamics a lot better. If he rips up this deal, the pressure will be on Iran to give him a better one. He doesn’t need grand international groups to agree to a list of weak demands- he just needs to scare away foreign investors. Meanwhile he can covertly pursue regime change. The Trump-led strike in Syria was also really part of a message to Tehran, as was the Israeli attack on Iranian positions in Syria, and the KSM diplomatic tour of the US.
Meanwhile moving the embassy to Jerusalem is no mistake. Rather it corrects the mistake previous Presidents made in giving Palestinian rejectionists a pocket veto. The whole approach you advocate was tried for 8 years by Obama and it failed miserably. The “peace process” didn’t produce any negotiations- it was dead. Trump’s transactional approach brings in the larger Sunni Arab strategic interest in forming an alliance with Israel to counter the ascent of Shia forces under Iran. The Palestinian ploy to undermine and destroy Israel has outlived its usefulness. Ttump is also putting on more pressure by cutting aid. His message is that the Palestinian side needs to negotiate in earnest and that time is not on the side of the rejectionists. The deal is going to get worse if they wait. It’s hard to know if this will work, but we know what came before didn’t work at all.
Finally pulling out of Syria is not necessarily a mistake. The real question is whether it serves the US national interest to be there in the first place. What are the goals? What is the strategy? Then can the case be made to the US public and can Congress vote for war in Syria? Obama tried low-level ineffective intervention that brought about prolonged wars of attrition and allied the US with some questionable extremists. An argument can be made Trump has an opportunity to get out now with minimal damage and remove US forces from harm’s way. Possibly that’s the best move, but it’s definitely not a clear error.