Plus, it's a property connected to a large lot I already own, and there's a premium in my city on selling combined lots in hot areas.As far as the bills, the property technically isn't a triplex - but it could be configured that way.

Existing home sale prices will be pulled up and up to and above this mark adjusted on geographic scarcity and configuration of home.

Depending on the lot, there could be many configurations to divide the outdoors space.

You may be able to get the city to allow it as a non complying triplex depending on how long it’s functioned that way, what the current configuration is, and whether or not from a building code perspective the units are have the correct fire separation.

I had a riverfront house with an attached garage apartment and could be configured several different ways; I had different listings for each size (to minimize confusion regarding how much of the house a guest was getting), and made it so you could book the whole house furthest in advance, then for closer stays there was the option to book the smaller version.  

We have always gone to 200 amp but then again 300 amp would definitely be a plus for a geothermal configuration, or the 240v 3x50amp breakers for a LARGE on-demand HW system or maybe the HVAC of a discreet 'grow' facility :) Admittedly the extra amps would adequately supply the future ADU BUT assuming it's allowed I would be inclined to install a separate meter/feed for that.The $3500 sounds reasonable...the last 200 amp upgrade I did myself ran about $800 in materials as those 'code required' AFCI beakers will 'break' you $$$.

Look around the neighborhood to determine how other properties are configured (bed/bath & square footage).

Just to make sure the construction and building configuration you have in mind isn't in conflict with setback or other restrictions as Luke mentioned.

In the homes that you have used this strategy, what was the bedroom/bathroom configuration? 

Depending on your investable capital 4-unit MFH in community configuration could be a good allocation as it allows better capitalization for supporting services (PM, Leasing, Maintenance).