Answer:
15,17,19
Step-by-step explanation:
Make an equation : 
Solve the equation:



Since x=15 then the second number is 17 and then 19
Hope this helps!
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So close! But instead of adding the multiplication sign after the number you need to add them before. Good job though! :)
3 and 60 are the extremes while 15 and 12 are the means.
The awnser is D but could also maybe be B