So Nac, what is the planet made of? Because if it is three times the size, it would have 1/9 of the gravity due to GM1M2/r^2. It must be heavy AS FUCK. Also, when you double the size of an animal it will need 2^2 times the diameter sized supports. This combined with three times earth's gravity either means these things are fucking walking slow tubs of meat that cannot more, or they have biologies so much better than any animal on earth it is fantastical.

Besides that, the issue with something that big is that RKKV [Ricky V's, if you're fucking hardcore] are too easy, and they will obliterate large battleships, because the energy is going to get there at some point, and anything that can move between celestial bodies at anything approaching a convenient speed has enough energy to obliterate them.

1/2(mv^2)

This right here fucks you over, and makes anything larger than a floating weapons platform worthless due to the sheer amount of energy being thrown around.