Revolut
Revolut's original 2015 pitch did not claim to want to build a global neobank. The SOM was surgical: European frequent travellers who paid excessive fees on currency exchange every time they used their credit card abroad.
The starting data point was verifiable: European banks applied an average 2-4% markup on every foreign currency transaction. With tens of millions of Europeans travelling regularly and using cards, this amounted to billions of euros in fees paid annually for a service that could have cost nothing.
Revolut's SOM was anyone who had travelled at least three times a year in Europe and had noticed the problem. A segment of a few million people — small as an absolute number, but highly aware of the problem and very inclined to word of mouth.
The SAM was all Europeans using cards abroad. The TAM was everything revolving around financial services for those moving across borders — remittances, international transfers, multi-currency investments.
Revolut built the SOM→SAM→TAM path sequentially. The credibility of the billion-dollar TAM arrived after the SOM was proven, not before.
The SOM is not "the market I can reach in 3 years" — it is the market I can capture with the resources I have today. It must be specific enough to have a first-name acquisition plan.