To see when they are 3 months if they have a slit or a cross is tricky I think. So sorry to be blunt, but we got our boy at 3 months and even then he had a testicle on the inside of each hind leg that we could feel (humans have all of their male equipment very close together, make rabbits bits are more “spread out”). Even before their testicles become very visible /descend you can feel a little lump on each side close to the back of their under body/ tummy, that the girls don’t have. Our boy is neatly 5 months old now and he won’t weigh more than 1.5kg. His testicles are each about the size and shape of a small mandarin segment and his penis, when he is in the mood is maybe nearly 4-5cm long.
They all hump each other for dominance so you can’t assume that this is a sign, coz our girl does it to her brother.