If you look at the "Addresses" tab (if you cannot see "Addresses", then use the menu item "Wallet -> Addresses")... you can right click on the address you want to send from and select "Spend From"...
As you can see in this example, while I have a balance of over 11.106 BTC, if I wanted to send from mnsp6mr6P42A3CfmEhNc35h1LwgQ5X7Hsj, I can only send around 9.9 BTC.
You can then enter multiple lines in the "Pay To" box:
Remember: You will only be able to send up to the amount of UTXOs (minus the required fee) in the chosen address. So in my example, I would need to have at least 0.039 + fee in my selected address to be able to send to the three addresses.
If you don't have enough coins in the address you want to use, you'll need to consolidate enough coins into the address you want to use first.
