Example, if I have a InstaMash its +2 rads and +1 hp(5s).

First, pretty common probably, is that 1 hp every second for 5 seconds or 1 hp collected total in 5 seconds?

Second, if I eat 3 of them, would I get +3hp(5s), or +1hp(15s)?

Third and last, do I even get the health from eating multiple foods at once. Like, if I eat an InstaMash and something else will the rest just be wasted?

1. for food I'm pretty sure that is 1hp per second for 5 seconds. If you are playing hard-core it differs between that function and instant with stimpaks (I think).

2. Be interested to know the answer to this one. I assume it is 1hp per second for 15 seconds. Or maybe even that any additional items of the same type consumed result in no effect until the first one has worn off. You can probably discern how this is working by looking in your Stats / Effects tab in your Pipboy

3. see above. I wouldn't be surprised if eating the second one while the first one is still 'in effect' is wasted.