Use two 2000w elements. You won't be able to power all three at once. 2000/120=16.6amps. 1375/120=11.45a. Powering both on one circuit will blow it. Too high of amperage. The way I do it is I have both elements in my HLT giving me essentially 4000w. I know it's a tad less but we will use that number for now. In the beginning I turn on both elements and heat my strike water to 175. I take my amount and add it to the MLT. Add my grains and stir. Throw the lid on and start the timer. Then I add enough water to sparge and set the temp to my mash temp. 15 minutes prior to Sparging, I'll stop recirculation and set the temp to 200. Once mash is up, i'll drain my first runnings. ONce this is complete, my water is up to sparge temp. I'll do my first sparge, mix, circulate and drain. Repeat for the second sparge. After this my HLT is empty and BK is full. Now if you plan on going electric with boil, youll have to look at using a Brutus 20 type design.
The other way is to keep the two outlets set for your elements. Use a selector switch to change where your current is being used. This would involve 4 total elements though. Two for your HLT and two for your BK. I do a hybrid system. Two elements and propane for the boil.