I have solved it myself, but L42 pointed me into the right direction.
In my table, data is located in J2, K2 and L2 and the sum is in M2 where I entered the final formula: =IFERROR(J2;0)+IFERROR(K2;0)+IFERROR(L2;0)
which basically means that I have manually added the 3 cells but checked for errors first, for each cell, with the function iferror and returned 0 if error exists.
Thank you!