If you can manage all the peripheral hours, AM and PM, they are both useful.
For MK, buying G+ is worthwhile if you'll be staying long enough to use a number of passes. I don't feel like G+ is worthwhile for AK, also questionable for Epcot. Look ahead of your trip at
MDE, though know the wait times one MDE are not trustworthy. They are often rather inflated.
It is sometimes hard to know when the waits are inflated, especially if you don't know what a 30min line looks like, or you can't see the line from outside the ride. The later it gets though, the greater the odds the wait is inflated. Sometimes the best option is to get in the line, give it 10minutes, and see how far you have moved. then decide if staying or bailing is the better option.
The key with a smaller child is to let them know up front that you might bail if the line isn't moving fast enough. On the other hand, it often pays to return to the line later, so let your daughter know that might also happen. Fair warning though, even adults sometimes get disappointed if we never make it back to a ride we enjoy.
Another way to get around buying G+ and minimizing waits is to skip 1 popular attraction in each park. Since your daughter is three, and maybe not tall enough for some rides, then skipping a ride or two makes even more sense.
Most parkgoers skip something when they visit MK. We rarely see CoP or HoP for example. they are fun, just hard to fit into our day. It often takes two days in MK to go on all the rides.
One way to zig when the crowd zags is to avoid eating lunch at noon.
