Top tier mons Gen 3

Yeah, Houndoom will probably be one of the top counters to Metagross, at least going into the battle blind without any knowledge of the movesets.

However, there still may be better options, such as Sharpedo, who also resists Psychic and Steel and has higher attack than Houndoom. Tyranitar is also likely to outperform, even with the weakness to Steel.

Machamp though, definitely a poor choice. The Psychic half of Metagross will neutralize his Steel half's weakness to fighting, plus Machamps's own weakness to Psychic as you've already mentioned.

Depending on the moves they will get - some newcomers like Blaziken and Swampert could make fine job against Metagross.

I'm mostly looking forward to Gardevoir. Having a strong fairy-type will bring some diversity. Also Aggron, but with his poor attack stat he won't be of great use. Blaziken could be interesting. Swampert will be outclassed by Vaporeon I think. Salamamce will be like Dragonite, nothing new here except he will probably get a flying quick move.

I'm mostly looking forward to Gardevoir. Having a strong fairy-type will bring some diversity. Also Aggron, but with his poor attack stat he won't be of great use. Blaziken could be interesting. Swampert will be outclassed by Vaporeon I think. Salamamce will be like Dragonite, nothing new here except he will probably get a flying quick move.