My husband and I have physical relationship once or twice a day everyday (except when I have my periods). In spite of that he always masturbates once everyday. When he asks me for the third time I always refuse. I want to know even after having sex twice is it Haram for me to say no? Is it wrong or right for him to masturbate after having relations twice?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
You are obliged to obey your husband fulfilling what he wants from you. Islamic Shariah does not set the limits of having sex to a limited number twice or thrice. As long as he feels sexual appetite help him to satisfy himself if you do not face any physical harm. If you abstain from having sex without any legal reason then you commit a major sin because Allah Ordered you to respond to your husband's demand.
It is not permissible for him to masturbate, if he does so he transgresses the limits of Allah, because Allah did not allow one to fulfill his sexual desire except with his wife or slave woman.
Allah Says (interpretation of meaning): {And those who guard their chastity (i.e. private parts, from illegal sexual acts). Except from their wives or (the slaves) that their right hands possess, - for then, they are free from blame; But whoever seeks beyond that, then those are the transgressors;} [23:5-7]
We advise the questioner to respond to her husband's demands, especially his demand to make sex to avoid making him angry or pushing him to indulge in Haram practices.
Allah knows best.