Not once it becomes a three-on-one. We could probably keep it up for quite a while as a 2-v-1. Adding a third realm to the mix is gonna hurt. There's not many realms that could hold off three realms of such combined size.
The whole Terran/D'Hara merger hurt Astrum, and caused a lot of political damage within the church. Had Terran stood their ground, Astrum would have stood a better chance of winning. Terran's job was to hold out as long as possible, and they gave up.
In Niselur, our job was to face the Astrum and Corsanctum army as long as possible to allow our allies down south to do their work. We had hoped we might win against Astrum, but as it turned out, Astrum won, but that was not our main objective. The looting in Niselur helped stabilize our control situation as we were facing massive protests, and was close to Darfix going rogue, when your raiders went across the realm looting and causing the peasants to hate Astrum and Corsanctum, and stopping the protests. We held on long enough to allow Asylon to complete their risky objective of doing two straight takeovers without a refit.
This was the first time that Asylon and Niselur worked together, so we had to trust in our allies abilities. Our trust it seems was rewarded. Asylon is the stronger of the two realms, but Astrum couldn't resist trying to take out King Leopold, and in doing so ignored the fact that Asylon is more experienced in warfare, and knew what need to be done to weaken Astrum.