First of all, you should distinguish between the fitness of the gene and the fitness of the people. Second, I am using as input the empirical observation that the fitness of the achondroplasia gene is 1/4. Third, and tangentially, you should distinguish between the fitness of the children and parents.

(1 gene vs parents) Let us consider the 3 surviving children. Out of the 6 copies of the gene, 4 are wild type and 2 are achondroplasia. But in the parents, half of the genes are achondroplasia. Thus, regardless of how many children the parents have, the fitness of the gene is 2/3 the fitness of the parents.

(2) Empirically, 1/4 of achondroplasia births are inherited and 3/4 are de novo. Assuming equilibrium, the gene is producing 1/4 of replacement fertility, so it has a fitness of 1/4. If dwarfs only reproduce with non-dwarfs, they, too, have a fitness of 1/4. But if they only reproduce with dwarfs, they have a fitness 3/2 of the gene, thus 3/8.

(3 parents vs children) The 3/4 you compute is the reduction in the proportion of pregnancies yield children. This is a kind of infertility, though more emotionally difficult. It is only relevant if the parents are trying to reproduce as fast as possible. In the modern world, parents usually target a small fixed number of children and infertility has little effect. In both farmer and forager societies, children were probably modulated to available food supply. Such a wasted pregnancy does not reduce the number of children by 1, but probably delays future children by a year. If the usual interval is 4, this might reduce fitness by 1/4. But the effect is probably significantly smaller. If people are reproducing at the optimal speed, taking into account risk of famine, a small perturbation probably has little effect.