What Pokemon would make for good starters?

[PiccoloX]

Stones permited, Nidoran would make for good starters given their diversive move pool. The majority of 3 stage evolutions that would make good starters are already found near home base, so those are rather pointless to list. Following the trend of single types evolving into multi types of Fire, Water and Grass through leveling alone the ONLY pokemon that falls under these rules is Tympole. There are plenty of multi type starters with 3 evolutions in Fire, Water and Grass, but only Tympole starts off as 1 type, has 3 evolutionary forms and evolves through leveling.

Every other 3 form single type in these categories evolves through use of stone, affection, breeding for a baby form or trading.

 

[TerraTF]

The Talonflame line, Machamp line and Roggenrola line seem to be the best

 

[Ghurlag]

Working within the typical Fire/Grass/Water setup, with the three-evolution restriction, the options from existing lines would be as follows.

Fire: A bit of an awkward group.
* The Fletchling line is perhaps the simplest choice, requiring no special items or trades, a Fire/Flying end result being attractive enough, but Fletchling itself is Normal/Flying and so seems to break the rules a bit.
* The Magby line is strong, perhaps a little too strong for a starter, and 'trading required' to get Magmortar seems a bit of an uncomfortable restriction. In the right game setup, it could be made to work.
* Finally, there's Litwick, a primary Ghost typing which requires an evolutionary stone to reach its third stage. Definitely an edge case.

Water: Plenty of near-fits, only one easy drop-in.
* Poliwag seems to grow in stages pretty similar to the typical starters, the only real hurdle being the need for an evolutionary item to reach its final stage. The ability to decide your own third stage would be distinctively unusual for starters.
* Lotad is fairly similar in principle, though without the multi-choice issue. It does however blurr the Water/Grass line, making it an unlikely pick.
* Tympole is a pretty simple pick, if a little dull due to its similarities with Mudkip, mitigated a little by a range of sound attacks. The real plus is the lack of obstacles to the third stage.
* Horsea would be a fine starter, were it not for the 'trading required' block on its final stage.
* Spheal could work, with no evolutionary blocks but an unfortunate primary Ice typing.

Grass:
* Oddish is a near-exact parallel of Poliwag, with a three-stage line which splits at the end using stones. The lack of this pattern in Fire is all that prevents a game setup based around such a mechanic.
* Seedot again is a mirror of Lotad, a reasonable starter line with an evolutionary stone block.
* Bellsprout is another contender for this evolutionary-stone role, though I would consider it a less interesting one.
* Hoppip could theoretically be used, and it has the advantage of no evolutionary block on its progression, but there are obvious drawbacks for a Lvl 5 start point.
* Budew could fit the bill, although it would face some evolutionary struggles, with the friendship-based first evolution and an evolutionary item second.
* As mentioned above, Lotad could slot in here, although its primary typing would make the choice questionable.
* Sewaddle would be an unusual choice, with a wrong primary type and a friendship-based third level.

Combinations:
* The only combination with 'normal' evolutions would be Fletchling, Tympole and Hoppip.
* A secondary-type version of the normal setup could exist with Litwick, Spheal and Sewaddle, but the method for reaching the final stage differs for each (which I guess could be integrated into a difficulty-selection idea).
* Horsea and Magby together would make 2/3 of a 'trade to reach your final form' game, which could be quite interesting. The lack of a similar evolutionary pattern in Grass somewhat breaks this, but could be remedied by having a traded Seedot/Bellsprout/Budew exposed to their evolutionary item by a trading NPC.
* Poliwag and Oddish make 2/3 of a 'choose your own final form' game, which again could be interesting, but is currently impossible due to the lack of such a pattern for the Fire type.

 

[HeroOfDarkness]

The Nidoran lines would be interesting enough. I could see a Pokemon Ranger game starting with the Male and Female having those two. (Like Plusle and Minun)