You will send from your Electrum desktop computer, and the amount being sent will be taken from the three addresses. If I am not mistaken, your oldest coins come out first. The receiving Bitcoin address (Phone wallet, etc...) will combine the coins into one address/transaction. For example if you transfer $75 to your new address...

Address 1: $50 > New Balance 0
Address 2: $50 > New Balance $25
Address 3: $50 > Remains at $50

Receiving Address: $75

Hope this helps.