Let the cost of each of the first three tires be \( x \). According to the deal, Sebastian buys three tires and the fourth tire costs $5. Thus, the total cost for the four tires can be expressed as:
\[ 3x + 5 = 237.50 \]
To find \( x \), we can first isolate \( 3x \):
\[ 3x = 237.50 - 5 \]
\[ 3x = 232.50 \]
Now, divide both sides by 3 to solve for \( x \):
\[ x = \frac{232.50}{3} = 77.50 \]
The cost of each of the first three tires is \( \boxed{77.50} \).