Retrofit single-zone hydronic baseboard to 3 bedroom zones with 3 way diverter valves.

Yes, It is a viable approach . I would try to fit the Diverter Valve and the new bypass pipe (to the next baseboard unit in the next bedroom) within the existing baseboard end caps. The bypass pipe can run below the finned element in each baseboard unit. Providing power to the diverter valves is another issue I'm not sure how to address but it's low voltage so it's easier than 120 VAC and its code requirements. The only other approach I can think of now would be to have each baseboard supplied directly by a home run from the boiler with a 2 way zone valve and a common return pipe. Don't know if you have easy access to run PEX tubing in place of copper ( a cost savings). You would need to build a small manifold at the boiler with 3 zone valves and 3 returns. All wiring would be located at the boiler and you could use smart thermostats like Ecobbe with a wireless remote temp sensor in each bedroom. This eliminates the need to run thermostat wires in each bedroom. Can you do this type of work yourself or do you need a heating guy? There are two costs: 1) for the piping and valves and 2) the electrical for the valves. Doing either one yourself is a big cost savings. My experience is that it's cheaper to just get a good plumber to do the work as you designed it rather than a Heating Contractor with his overhead costs. PEX is easy to work with and much cheaper than copper. I assume you need to have big differences in bedroom temperatures to justify the cost of this work. Do you have un-occupied bedrooms that can have a low temperature? A rough ball park cost would be about $1000 in material ( valves, transformers, thermostats, pipe and fittings) and maybe $2000 in labor( based on your location). There is one negative factor in that you may have a lot of boiler short cycling because of the small size of each zone. In the ideal world, you would install buffer tank to reduce the short cycling. Also, how is your insulation in your 1980's home. If you are looking to save on heating costs with this zone design, would the money be better spent on improved insulation and windows. Just a thought. Good Luck.

You either need a buffer tank or a modulating boiler because of the microzoning.

TRVs might make a whole lot more sense than zone valves and thermostats.

Doing a heat loss calculation and sizing the radiation to be balanced might be a much simpler solution. I bet you have a series loop and they just put the baseboard wherever they could instead of sizing for the decreasing water temp in each room further in to the loop.

I would think all the 3 way valves would add up to a large Cv and be restrictive. I agree with 2 way valves but with 3 bedrooms on 1 floor do you really need this?

Are the temps that uneven?