Merefolk, Elves, Goblins, Spirits they all have +1/+1 lords that give their tribe another feature (islandwalk, mountainwalk, haste, hexproof etc...). But the dragon's just don't have an efficient lord.
As a person that was looking through options for a dragon tribal deck recently, an option that needs to be considered is making a blue-black-red dragon tribal, and that way you can use Silumgar, the Drifting Death as a basic win condition (the -1/-1 counters really make it an anti-token, and just imagine the control you could use with blue too, but also imagine full-swinging with a board of dragons, assuming you maintain control). Also, it adds a whole new revenue of different dragons to use, like [Dragonlord Silumgar, and other dragon-related cards (for example, Bladewing's Thrall or Dragon Shadow) that would just make things a lot easier for those dragons you have.
As a person that was looking through options for a dragon tribal deck recently, an option that needs to be considered is making a blue-black-red dragon tribal, and that way you can use Silumgar, the Drifting Death as a basic win condition (the -1/-1 counters really make it an anti-token, and just imagine the control you could use with blue too, but also imagine full-swinging with a board of dragons, assuming you maintain control). Also, it adds a whole new revenue of different dragons to use, like [Dragonlord Silumgar, and other dragon-related cards (for example, Bladewing's Thrall or Dragon Shadow) that would just make things a lot easier for those dragons you have.
Here's an example of a deck, albeit black-blue, it just goes to show the amount of options you would have expanding the deck to tri-color:https://deckstats.net/decks/118141/1119066-silumbar-sweeping-death-neg-co/en (I'll see if I can update this later for a better example with the three colors later)