+3 is ${\color{Red} 0}011$ in sign magnitude where ${\color{Red} 0}$ indicates positive.
-3 is ${\color{Red} 1}011$ in sign magnitude where ${\color{Red} 1}$ indicates negative.
when we add them in sign magnitude,
011 + 011 will give 110(magnitude) and adding ${\color{Red} 0}$ + ${\color{Red} 1}$ we get ${\color{Red} 1}$(sign bit)
=> the number is ${\color{Red} 1}110$ which is -6.