You have it reversed, iirc. The iron in broccoli is one of the best in terms of plant based bioavailability, and has a higher base amount, and the iron in lentils has some of the least amount of bioavailability, around 2%. Spinach's bioavailability is low too, but still higher than that at 6%.
Of course, in practice, you can just take an iron supplement or cook with cast iron and not worry about this at all.