In Burning Crusade era 25 man raiding, 3 tanks were the norm. That's actually how I got my in with a Tier 6 guild. I was their 3rd tank. It wasn't a great job, especially in the days before dual spec, but it was good first rung on your way up the tanking hierarchy in a guild. The idea has returned with the penultimate boss of the current Highmaul raid, High Imperator Mar'gok.
"3-tank Heroic/Mythic Imperator Mar'gok"
High Imperator is a crazy 5 phase fight that challenges all groups even down to LFR. Every even phase is an 'intermission' where Mar'gok becomes unattackable and the raid deals with a set of adds. In the second intermission (or 4th phase) an additional add called a Reaver spawns. The Reaver is a melee and hits like a truck. He needs a tank. At the same time this is happening the two tanks still need to pick up the Warmages and Volatile Anomalies. The Reaver also periodically knocks his tank back and drops aggro. Handling all this feels like trying to dance while juggling chainsaws.
Some guilds have elected to bring a 3rd tank to the fight. The 3rd tank allows the other two tanks to concentrate solely on their adds while the 3rd tank takes care of the Reaver. It's important this 3rd tank does a reasonable amount of DPS during the early phases where she isn't needed to tank. The DPS requirement on this fight isn't so tight that the small amount of DPS you lose for bringing the 3rd tank shouldn't be what wipes you. This is more of a survival fight than a DPS race.
Another application of the 3rd tank would be Brackenspore. Two tank can rotate the Rot debuff while the 3rd tank handles the big add that spawns. This way your tanks never have to deal with the add hitting them plus the Rot debuff ticking at the same time.
I thought about how I would pull this off but, as usual, my friends at Maintankadin had it all figured out. "DPS as Protection Spec"
Glyph load out
I changed my Glyphs to Harsh Words, Focused Shield and Final Wrath.
This build is more single target focused. The options for multi target would Double Jeopardy assuming you were good at cycling between targets every Judgement. Alabaster Shield would be the other option, but it would seem like that counteracts Harsh Words. If you are spending Holy Power on Harsh Words you wouldn't be building any Alabaster Shield stacks since you wouldn't be using ShoR.
Holy Avenger / Sanctified Wrath
Holy Avenger and Sanctified Wrath sim out pretty close but I went with Sanctified Wrath because I feel like it easier. With Holy Avenger you really have to line it up perfectly with Seraphim and I don't trust myself to pull that off just yet.
Initially, you would run Seal of Truth in this build instead of Seal of Insight. But after the nerf, Seal of Righteous does more DPS (even to a single target) than the Seal of Wet Noddle Truth. When the transition to the 4th phase is close, switch to Seal to Insight and replace Harsh Words with Shield of the Righteous in the rotation.
If I can pull this off, not only would it prepare me for a role on Mar'gok, but it would also allow me to use this spec for soloing and I could try swap my Ret spec to try out Holy once I fill in the Holy specific gear slots (weapon, shield, trinket, back and ring).