The summer discounts are not out yet. They typically come out in April or May at the latest. The spring deals end April 14, so I imagine the 5 for 3 will be out soon after if they are doing it this year. So be patient. The reason why expedia is quoting you the same rates as Disney is that the way Expedia and Orbitz get you the discounted rates is to wait unil Disney is done selling them. (this usually happens a month or so before travel, but it really varies--could be 3 weeks, or a little over a month). The Disney website will show "sold out" on their website for rooms, but when you go to Expedia and Orbitz they will have the rooms there at a discount. Obviously this is risky because Disney only has these rooms to unload if there are excess rooms. They sell them all to these discount places (sort of like TJ Maxx for hotel rooms) some of the time. If Disney is effective in selling them, then they don't go to the dicounter. Hope that is clear.
Don't stay at the Dineyland Hotel this summer--because it is under massive construction in the pool area. The new pool area will be open NEXT summer, not this summer. Even if you don't swim it matters because it messes with the overall environment and should get what you are paying a premium for.
If I were you I would stay either at Paradise Pier or the Grand Californian if you can get a good rate. You can still make a reservation at the Carousel Inn now, and cancel it later if you don't get the rate you want. I believe it is worth it to stay on property, and not just for the MM. This trip is a big deal as you have never been before, and likely won't be back soon. Make it as memorable and good as possible. The Paradise Pier actually has some really competitive rates compared to the other two hotels, and people seem to get upgraded to nicer rooms there easier than the other hotels. My friend booked a standard room there and got an amazing high tower theme park view and could see World of Color from her room. They were pleasantly surprised by the PP, as have always been GC diehards.
What made you decide on the Carousel Inn out of curiousity?