Shouldn't you be comparing the total amount of damage done by each? Assuming you can buff each one every round, the Golem starts at 5 (or 7) and increases by 2, while the Steam Machine starts at 0 and increases by 4 the first time, then effectively 3 each round afterwards (because the damage is done before the counter is removed, so +4, dmg, -1, +4, dmg, -1, etc.).
Observe (turn 0 is when you play the card):
Turn | SM | Dmg. | LG | Dmg. | LD | Dmg. |
0 | 0|8 | 0 | 5|1 | 5 | 7|1 | 7 |
1 | 4|8 | 4 | 7|3 | 12 | 9|3 | 16 |
2 | 7|8 | 11 | 9|5 | 21 | 11|5 | 27 |
3 | 10|8 | 21 | 11|7 | 32 | 13|7 | 40 |
4 | 13|8 | 34 | 13|9 | 45 | 15|9 | 55 |
5 | 16|8 | 50 | 15|11 | 60 | 17|11 | 72 |
6 | 19|8 | 69 | 17|13 | 77 | 19|13 | 91 |
Look carefully: the gap between the Steam Machine and the Lava Golem goes +5, +8, +10, +11, +11, +10, +8, and so on. So eventually, the Steam Machine will have done more total damage than the Golem. The gap between the Steam Machine and the Lava
Destroyer goes +7, +12, +16, +19, +21, +22, +22, and follows the same pattern. If you chart these out, you'll find that the Steam Machine eventually does total more damage than both, but by the time it does, the game will have ended long ago. You're better off with the Lava Golem/Destroyer either way.