So did everyone else: https://en.wikipedia.org/wiki/Flynn_effect
3 points per decade = 15 points per 50 years. So this shows that immigration to the US had no impact on the IQ of this group.
Whole developed world has very similar IQ levels to United States, except for China, Japan, Korea which have higher average levels: https://iq-research.info/en/average-iq-by-country