Click on the micro:bit icon on your toolbar or go to https://makecode.microbit.org/ on your browser.
Click on "New Project". The MakeCode editor will appear
Delete the on start and forever blocks by dragging it to the toolbar
Click on the pink Input category in the toolbox
Drag the on button press block and move it to your workspace
Your workspace should look like the image below
Find the show string block under Basic
Drag the show string block and move it inside the on button pressed block
Type in 1 in the show string block
On the simulator side, press the A button and you will see 1 display
Drag another on press button block to your workspace
Click on the drop down by A and you will see 3 options displayed.
Select B from the dropdown
What this means is that when you press button B, something will happen
Drag a show string block to the on button button B pressed code block.
Type 2 in the show string code block.
On the simulator side, press button B and the LED should display 2
Once again, drag the on button pressed code block to your workspace
Select the option A+B from the drop down
Drag a show string code block to the on button A+B pressed code block.
Type 3 on in the show string code block
In the simulator, press the A+B Button and the LED should display 3
Save you code as 1 2 3 and press save to download.
Drag your code file named microbit-1-2-3.hex to the MICROBIT(D:) folder to upload your code to your micro:bit.
Test your micro:bit by pressing Button A, then Button B then both button A and button B at the same time.