At that stocking rate, you should be fine as long as you monitor the ammonia & nitrite. Feed only lightly and do not add any more fish until there is measurable nitrate and no measurable ammonia or nitrite. Even if it seems all is fine, wait at least 6 weeks before you even think about adding more fish. When your 3 get to be 18", the pond will not look so empty. When they are 22", it will look crowded.
