Triangular numbers are 
When we arrange dots equal to numbers 3, 6, 10 we get triangles such numbers are called triangular numbers or triangle numbers. Here each subsequent number in the sequence adds new row of dots to the triangle. To use the triangular number we follow pattern keep on adding the next natural number.
For example: 1+2=3
1+2+3=6
1+2+3+4=10 and so on. So here 3, 6, 10 are the triangular numbers.