(1) The beliefs of Tabligh Jamat are same as of Ahl Sunnah al-Jama’ah.
(2) Going to jamah for 3 days or 40 days is according to a system. It is also for the reason that these days [3 days or 40 days] have great importance in self reformation and purification. Many Hadiths speak for special rewards on doing some deed regularly for 40 days. Specification of these days [3 days or 40 days] is not some addition in the religious affairs, so it cannot be termed innovation.
(3) It is not necessary to go to jamah. However it is mandatory to seek knowledge as much it is required and self reformation is also mandatory. In order to achieve the said purposes going in jamah is one of the useful methods.
Allah knows Best!
Darul Uloom Deoband