If you only have 3 units, you are not ready to siege a city. That is kind of the point.
Not 3 units.
3 armies.
That's 7*3 = 21 units. Mix of horsemen, defenders, and archers.
Following that is my hero force: 2 heroes, 2 archers, sand golem, and 2 defenders.
Problem is that, besides my 2 heroes, everything is fresh off the line and can't stand up against the 3 milita, 2 archers, 1 hero, 1 wounded hero, 1 defender and 1 catapult.
Only the Defender can be permanently destroyed. All others respawn to full HP for each battle (the heros don't obvious get hp back, but all they need is one hit to knock out one of my troops).