One misconception you have is that 3 players = 225%. It's actually 200% (4=250%, 5=300%, etc).
TF2 constantly checks throughout the uber to find the drain rate -- once a flashed player is no longer ubered, your drain rate returns to normal. Specifically, the game checks for the number of players that are flashing and you're not currently healing (note: your drain rate increases if you uber 1 person then break the beam as if you had just flashed someone). In your hypothetical situations, the drain rate is 100% when you pop on the pocket, 150% for 1s while the roamer is flashed, then 100% after. If you give the beam to a third player, it increases to 150% for their flash (or 200% if the roamer is still flashing) then goes back to 100% once no-one is flashed.
Perhaps an easier way to look at it is that you have 8 seconds of uber + 1 second flashing at 0%, and every time you flash you lose 0.5s.
Here are some more details:
1. The uber drain rate is 12.5%/s while you have 1 target ubered and are connected to him.
2. For each player who is flashed as a result of your uber, the drain rate increases by 6.25%/s (a flash lasts 1s). So, 18.75% with 2 players ubered (1 connected, 1 flashed), 25% with 3 players ubered, etc.
3. When you hit 0% uber, you and your current target are flashed for 1s.
My wording is specific here for a reason: these are some things I didn't realize until I went tic-by-tic in a demo of me ubering different patterns of bots in spawn. The meaningful in game consequences are:
1. If you uber and your pocket jumps out of heal beam range, even if you do not flash anyone else, your drain rate increases to 18.75%/s for 1 second or until you reconnect with him. You get longer ubers if you're connected the entire time, but since that's often impractical, the result is that you have a half-cost flash when your pocket jumps (since you'll be draining faster anyways -- the only penalty is when you switch back and whoever you flashed is causing your drain to increase).
2. Right before you hit 0%, you have a free flash. Why? Examine:
-You have 5% remaining before your uber is out, with one target ubered.
-You switch targets. Your original target is now flashing for 1s, and your drain rate is at 18.75%/s.
-Before you hit 0%, you attach to another target. Due to fact 3 above, you and your new target and now flashing for 1s.
Hence, you can have an 8s uber on 1 target, then 1s with 2 players ubered right at the end. From what I've seen, almost all experienced medics do a flash right at the end of their uber, but I don't think many consciously realize why it's a good idea (I had the habit before I went and found these results).
In case anyone cares, the 3 facts above also apply to the kritz drain rate. You don't get flashes with kritz (hence fact 3 is kind of a moot point), but you still have that increased drain as a result of switching (bleh). Still well worth it to switch a kritz though, since typically you shoot 1-2 crit stickies, then 15 scouts and 22 soldiers are on your head and a crit rocket is the right tool for the job.