It's not at all advisable to let your Ruby have out of cage time when the dog and your cats are sharing the same space.
Even if your cats are mellow, it's important to realize these are irrational animals who can and will act by instinct. If your Ruby is even playfully taunting the dog and your cats, this isn't a good sign as it will only be a matter of time till the dog you are temporarily taking care of and your cats to give in to their natural hunting instincts and chase your Ruby with the intent of harming him.
You have no idea of the number of accidents we have witnessed here at Talk Budgies related to dog and cat attacks on small pet birds which ended in tragedy. Even the smallest superficial scratch done by a cat can prove fatal for a budgie.
Not only is it important, but it's also advisable that your Ruby's cage is in a safe place where the cats (and dog) aren't allowed access. There have been situations where cats managed to get budgies through the cage's bars, by using their claws. Also when having out of cage time, the cats and dog should also be denied access to the room Ruby is in.
For the detailed info and measures to ensure the safety of your budgie, please check this link: https://talkbudgies.com/general-budgi...irds-prey.html