No, if you claim legal rights, you lose your legacy in the will whether heritable or moveable. I hope that answers your question. Please leave a positive rating so that I am credited for my time.
From what you say, the will provides that three children share the house three ways but the moveables are bequeathed to only two of them. If the third child decides to claim legal rights in the moveable property he will get his legal rights entitlement to the moveable estate but he will lose his entitlement to a share of the house under the will. In Scots law you can choose to take what you are left in the will OR you can claim your legal rights but not both. Whether you should take what you are left in the will or choose to take legal rights depends on what each are worth. Legal rights where there are three children and no surviving spouse would be one ninth of the moveable estate. If that is of a lesser value than the one third heritable estate in this instance then it would be unwise to claim legal rights. I hope that makes it easier for you to understand.