The real issue of not having 3 clubs is down to there not being enough players to have one competitive team, let alone 3. Last year there were only 2 teams and they were the 2 worst in the First Division by a fair way.
Mervue seem to have made a fair stab at establishing themselves as a LoI club but only represent a tiny area of the city and haven't really tried to develop further. Whether this is sustainable is questionable.
Salthill Devon appear to have little to no interest in the LoI as a whole. Look at their website any weekend and you'll struggle to find any mention of the LoI side and no promotion of games. Last season they said they'd have a 1 year trial with their LoI wing rebranded as SD Galway and playing in Terryland. They put this proposal to their members a week after their opening game to the season.
During the season they didn't promote the games around the city and didn't seem to have any appetite at building a supporter base.
To me it seems Salthill are in the league because Mervue joined. Now they probably feel that by hanging in there they can get some sort of pay-off as the FAI will need them in agreement to facilitate a new club.