None of this is speculation.
The third Civ. will be the Celtics.
After some data mining from the recent beta patch, I discovered some information, for example...
<Tech name="CeltCapAge4" type="Normal">
<DBID>4679</DBID>
<DisplayNameID>59090</DisplayNameID>
<Cost resourcetype="Food">50.0000</Cost>
<Cost resourcetype="Wood">1000.0000</Cost>
<Cost resourcetype="Gold">1000.0000</Cost>
<Cost resourcetype="stone">600.0000</Cost>
<ResearchPoints>60.0000</ResearchPoints>
<Status>UNOBTAINABLE</Status>
<Icon>UserInterface\Icons\Techs\AgeUpL4_ua</Icon>
<RolloverTextID>59089</RolloverTextID>
<Flag>AgeUpgrade</Flag>
<Flag>IsAward</Flag>
<Flag>AlwaysShowButton</Flag>
- <Prereqs>
(Do not take this as a complete list, this is like the pre-alpha for the celtics, in data form) For example. Their units will be...
Carpentom - A Warchariot, not sure if it will be like a Hippikon, but I assume it'll be like a Sarissophoroi, based on history.
Druid - Equivalent to a priest
Spearman - Spearman (derp)
Woad Raider - like Hoplite
Swordsman - like Hypaspist
Champion - Like the Seamen Chieftains, basically an anti-archer infantry.
Skirmisher - Peltast equivalent
Bowman - Bowman, but with a shield (on their back, like hypaspists).
Horseman - Prodromos equivalent
---Unique Units--- (I know each Civ has 6 but they obviously haven't even put in any data for a sixth)
Ram
Gaesatae
HeadHunter
LongBowman
Warchariot
Edit:I'll add a list of the listed Advisors later.