riddle - Piece de Resistance - Nine is Three Tripled

Nine is Three Tripled

_{This puzzle is part of the "Piece de Resistance" series. Go back to Part 1 (Ace) for the story.
Ace Two Three Four Five Six Seven Eight Nine...}

Here is the transcription of the card...

Phase I - Truly. Tribond.

Bubonic
Egypt
Dogs

Phase II - Insert! Interpretation?

My leader was once, in short, an elementary British theatre.
My fellows achieve, register, and branch.
My tail is a Shetland fiddle.
And Quebecois use me to insert!

Phase III - Solution - Stepping Stone!

Hints

See Edit ;)

3, ..., 9

The whole thing has been solved. Anyone care to answer it?

Answer

Encouraged by the third hint I'll give a try

Phase I (solved by @Deusovi)

Tribond riddle:
- bubonic plague
- ten plagues of egypt
- plague dogs

Phase II - Insert! Interpretation?

Riley riddle

My leader was once, in short, an elementary British theatre.

An 'elementary' British theatre could be the London PALLADIUM.. (as suggested by @Stiv)
Palladium has the chemical element symbol "Pd"
From Omega Krypton Comment: Palladium used to be Pl in the early 19th century.

My fellows achieve, register, and branch.

log: a register is a log, a log can branch too

My tail is a Shetland fiddle.

A Gue

So we have Pl-log-gue -> Plogue

And Quebecois use me to insert!

the french word "plogue" means "plug", to plug in means to insert

Phase 3: Solution - Stepping Stone!

Stepping stone = word ladder

Looking at the source code of the question we find "Look out for capitals."
Well, having two words ending with -gue we find the capital of the Czech Republic, Prague (thx to @JS1)

Phase 1: Plague
Phase 2: Plogue
Phase 3: Prague
So 3 -> 1 -> 2 is a word ladder

Solution

Prague

Bonus

A stepping stone is needed to pass a river, gue in french is a small passage to cross a river.

Title - Nine is Three Tripled

Riddle 9 is divided into three parts