According to the semi-official timeline INRA defeated the Thargoids with the mycoid virus in 3193. Alliance/Thargoid peace was established in 3253.
The timeline where I get the year 3151 can be found here: http://wiki.alioth.net/index.php/Chronicles_of_the_Milky_Way
The sources that support 3151 being the correct year the Thargoids were defeated are as follows:
Hughes D. (n/a). 'Elite: Far Colonies Network - 3: Theory'
(1995). Frontier: First Encounters journal number 0007. Frontier Developments.
(1995). Frontier: First Encounters journal number 0432. Frontier Developments.
(1995). Frontier: First Encounters journal number 0470. Frontier Developments.
(1995). Frontier: First Encounters journal number 0545. Frontier Developments.
The page where the timeline is found acknowledges the conflicting year of 3193, and has this to say on the matter:
There are conflicting dates given over the actual year that the Thargoids were defeated during the 3100's. David Hughes (www.hughesd.co.uk) and the FFE journal articles both claim 3151, whereas Dylan Smith (www.alioth.net) has a later date of 3193. As the official FFE game journals identify 3151 as the date that the Thargoids were eradicated [14.7], that is the date that has been used in the Chronicles.