So if I want to stay in my home resort, it could be 1 week in the 3 bedroom, stay 2 weeks, 1 in the studio, one in the 2 bedroom.
Yes, that's right. The two reservations are completely independent and you can use or book them anyway you like. You can do this in one phone call (as you split the lockoff and pay the $80) or split them and book them separately (or deposit one etc...)
I could stay a week in the 2 bedroom and trade the studio in II or FC for a studio elsewhere.......
Once the units are locked-off, they are two completely separate units. Reserve one or both for use. Reserve one and deposit one. Deposit both. If you want to make a Florida club reservation (with one or both) you can do that as well. Your unit can be reserved/deposited up to 12 months out (assuming you only own 1 Marriott). The Florida club reservation can be made at 6 months out (7 months if you own two Marriott weeks etc..). I booked my studio into Beachplace at 5 months for March spring break week and then deposited. Good trader
Is a 3 bedroom likely to get a smaller unit in a prime place if we can travel off season and in a 1 bedroom, if we chose to deposit with II?
I'm sorry; I don't understand your question fully, but... The MGR 3br is a very powerful trader in II and so is the 2br. You would probably see a desired exchange quicker or in a more prime week with 3br vs 2br, but both are very strong. It will 'see' most any Marriott available (and lots of other very good properties as well). A Marriott deposit in II has 'Marriott Preference' and sees other Marriott deposits with (up to) 24 days of preference before non-Marriotts (in most cases). The 3br should show studios, 1br, 2br, 3br, 4br etc... If you lock-off the units, the 2br portion of the lockoff will show many Marriotts (maybe not all) depending on the strength of the week that you book. If you reserve your 2br Marriott lockoff in a very high TDI week before you deposit with II (you should always pick your own week--ask TUGGERS for help picking if needed) it will show very strong exchange weeks (studio, 1br, 2br, 3br). If you also deposit your Marriott studio in II, it will show Marriott trades with preference, but you will not see all of the weeks that you will with the 2br or the 3br. In flex, you will (probably) see the same thing with both 2br and studio. Flex normally has Marriott pref (group opinion) but only for a few (2-3) days.
Today, with my Marriott 2br MGR deposit, on instant exchange in II, I can see Marriotts in Spain, Caribbean, Newport, Hawaii, Palm Desert etc... Pretty much any where you want to travel--especially in shoulder season. The on-going search (OGS) would be much stronger, given enough time.
A third possibility you didn't mention: Lock-off, deposit both units in II, book a nice 2br in a prime resort with the 2br and then book another week in the same resort or another very close by (think Hawaii) with your studio via OGS or a quick grab during flex. Or, grab a nice unit that you can afford to travel to on short notice with the studio during flex--there are no size limitations to flexchange.