So can I just soft reset before evolution to get a 3 segment dudunsparce or maushold or is it already determined before evolution and I just have to catch a bunch and hope. And, if there is, how do I know which it evolves into.
Dudunsparce has two different forms: the Two-Segment Form and the Three-Segment Form. The form Dunsparce evolves into is determined based on its encryption constant (an extension of the personality value), with a 1/100 chance of becoming Three-Segment Form. This is not the case for wild Dudunsparce, which are always in Two-Segment Form regardless of their encryption constant value. If the Encryption Constant Mod 100 is 0, the resulting evolution is Three-Segment Form. Any other result yields Two-Segment Form.
Dudunsparce has two different forms: the Two-Segment Form and the Three-Segment Form. The form Dunsparce evolves into is determined based on its encryption constant (an extension of the personality value), with a 1/100 chance of becoming Three-Segment Form. This is not the case for wild Dudunsparce, which are always in Two-Segment Form regardless of their encryption constant value.
If the Encryption Constant Mod 100 is 0, the resulting evolution is Three-Segment Form. Any other result yields Two-Segment Form.
Maushold has two different forms: Family of Four and Family of Three. The form Tandemaus evolves into is determined based on its encryption constant (an extension of the personality value), with a 1/100 chance of evolving into the Family of Three form. This is not the case for wild Maushold in Tera Raid Battles, which have a form determined by the raid's difficulty rating (Family of Three for 4★ or Family of Four for 6★) regardless of their encryption constant value. If the Encryption Constant mod 100 is 0, the result is the Family of Three Form. Any other result yields the Family of Four Form.
Maushold has two different forms: Family of Four and Family of Three. The form Tandemaus evolves into is determined based on its encryption constant (an extension of the personality value), with a 1/100 chance of evolving into the Family of Three form. This is not the case for wild Maushold in Tera Raid Battles, which have a form determined by the raid's difficulty rating (Family of Three for 4★ or Family of Four for 6★) regardless of their encryption constant value.
If the Encryption Constant mod 100 is 0, the result is the Family of Three Form. Any other result yields the Family of Four Form.
Bulbapedia (Dudunsparce) Bulbapedia (Maushold)
As encryption constants are parts of the personality value, and personality values are created immediately upon a Pokemon generating for the first time, you are unable to soft reset evolution until you get the desired form.
Maushold and Dundunsparce forms are determined by their Peronality Values, which are determined when the Pokémon first spawns into the game. Which means you would have to constantly find a new Dunsparce and Tandemaus to successfully hunt for 3 segment and Family of 3 Maushold.
Hope This Helps.