This can be easily done via regular Multiplayer but if you have 3 Dummy accounts your want to try your luck with Bots you can try this method.
Open the command console: ESC > Gear Icon > Under the keyboard tab select Advanced > Check off Enable Developer Console
Press the ` button to open the console and type:
map cp_dustbowl; hostname "boost"; sv_lan 0; sv_cheats 0; (I had to input this twice to be detected on my LAN for some reason)
That will create your match. Choose Engineer on the Red Team. Then type the following command:
mp_teams_unbalance_limit 0; mp_idledealmethod 0; mp_idlemaxtime 0;
You can add bots by using tf_bot_add red or tf_bot_add blue
You can alse set them to use melee only: tf_bot_melee_only 1;
Build a dispenser and try to get the Bots to heal there or just let your dummy accounts heal there. You should be able to damage your dummy accounts using fall damage or Bots
If you choose to do this via Multiplayer. Build a dispenser and you will be able to move it around using left click on the mouse. Take it near the front lines and people will run back to heal or ammo up. You can also place it near another engineers dispenser and that should get you healing the other engineer at times.