Back $2$ the basics !!

Two pea $\text{(Pisum Sativum)}$ plants with heterozygous genotypes (for all characteristics mentioned in the problem ) are crossed. The first plant has purple colour, round seeds and yellow seed colour and the second has purple colour, wrinkled seeds and green seed colour. Find the probability that atleast two recessive traits are exhibited (in the progeny of the resulting trihybrid cross). If this can be expressed as $\dfrac{a}{b}$ for $gcd(a,b)=1$ , find $a+b$ .