confint()function is aplicable for a fitted model object, not a numeric vector. I think you are trying to get a confidence interval for your mean, for a simple approach you can use a normal distribution, something like this.
filtered_data <- your_data$head_circ[your_data$exp_group == 3]
m <- mean(filtered_data, na.rm = TRUE)
s <- sd(filtered_data, na.rm = TRUE)
n <- length(filtered_data)
error <- qnorm(0.975)*s/sqrt(n)
left <- m-error
right <- m+error