In alpha decay a radioactive isotope releases a helium nucleus with mass of 4 and charge of +2 and a new element, with a lower mass in produced. Thus, U- 235 if it released an alpha particle would form element 88 (Radium) with a mass of 231. In beta decay, a radioactive isotope releases an electron which hass no mass and a charge of -1. Thus, if Pu-244 decayed by release of a beta particle, a new element of mass 244 and atomic numer 95 (Americium) would be formed.
In fission, a uranium nucleus is split by a neutron into two smaller elements along with the release of additional neutrons and energy. The neutrons released can split additional uranium nuclei resulting in a either a controlled chain reaction (nuclear energy) or an uncontrolled chain reaction (atomic bomb)