goldfish jealousies

I have two small ponds and i put two goldfish each pond a few years ago.
one died so i put the newly single one in with the other two.

so in the one pond i have three fish. the original two in that pond which
comprises of the biggest (a) and one a bit smaller (b) and the one newly
moved from the pond (c) which is much smaller than either a or b.

when (c) first went into the pond is always stayed close to a and b and so
the three always swam around together. today we have just noticed that
both b and c are attacking a.

(a) has got a bit of white fungus on it and is either heavily pregnant or
has some illness that causes the stomach to swell. it doesnt seem all that
well because it moves about fairly slowly.

since it was being attacked i have moved it into the other pool so it is by
itself. do you think it's some sort of ' love trangle' violence or because
it
is ill (or pregnant) that causes it to be attacked by both fish? any
advice for a novice please?