I don't know for sure how this is arranged actually. There are two possibilities I think.
If you fully MU your Hornet m2, you'll get m3 for free. You then already have Hornet m3. The price of a kit remains the same if you already own one or several parts of it. If you fully MU your Hornet m2, you'll get m3 for free, meaning a 100% discount on Hornet m3. The kit will then have a 100% discount on Hornet and a 35% discount on both Railgun and Prodigi.
I'm not sure if you have the option the 'buy' Hornet m3 for free when having a fully MUed Hornet m2 or if you just get it if you've a achieved the necessary rank. Therefore I'm not sure which of the two options I explained will be the case.