i want to send 3 btc to someone, but the remaining 1 btc i dont want back to my Core wallet but i want to transfer it to my electrum, so if i use my electrum address as a change address in Core will it go to my electrum?
Although you can use a change address for this, the proper way to do it is use "Add Recipient", and add your Electrum address there. You can tick "Subtract fee from amount" so you don't have to subtract the total fee.
I have 5 btc in Core wallet, i want to send 3btc to Alice and 2 btc to Bob, to make it all in one transaction can i send 3btc to Alice and use Bob`s address as a change address, thus Alice will get 3btc and the remaining change of 2btc will go to Bob.
I wouldn't do this, you risk sending your entire balance to the change address if you have more funds in your wallet, especially if you don't use Coin Control Features.
TL;DR it works, but you should know all the risks.