I think this can vary greatly based on the renters not only in a particular city or zip code, but right down to a neighborhood.
Generally speaking, most investor convo around this is still thinking in terms of a nuclear family and it is possible to find places where that is not your demographic. In the areas I'm investing in, hipsters returning to urban cores and looking for something akin to the condo vibe without the condo fees are buying 2/2 and 2/1s.
With that said, a 3 bedroom place is certainly going to give more options than a 1 or 2. I get great cash flow out of my 2bed 2 bath duplex, usually with younger couples as tenants. But as babies start to grow and multiply, that nice family has to seek something larger.
I also anticipate that I'm going to be marketing that 3rd bedroom on my current rehab as a home office now that many more people are working from home.
as to your heuristics,
pick your area
pick a sq ft range eg 1000-1300 sq ft
get an agent to compile a couple years' worth of sold data - in spreadsheet form that you can sort.depending on your local MLS rules, you may be able to do this yourself on Redfin.
with a little practice, you'll be able to find the properties that sold " as is" then went back on market after flip.
look for the ones that added bedrooms or bathrooms to the existing square footage.then look at those that flipped without changing the room count. Compare the sold price per square foot on the 2bed versus 3 bed. Then you'll have an idea for what adding that 3rd bed is worth in your target area.
man that post was too long. Sorry.
Id be happy to email you a copy of a spreadsheet for an area I've looked at to practice with.