I have written this script it print same line multiple times. what to do if I want to print it only one time?

BEGIN print "Average of salary"
cnt=cnt+1
total=total+$3
avg=total/cnt
END printf "Number of records:%d avg is:%d",cnt,avg

The problem is that you didn't enclose the below code within braces as following and awk will meet those as True statement and print every line it read:

 cnt=cnt+1
 total=total+$3
 avg=total/cnt

But instead you could do something like:

awk ' avg=(total+=$3)/NR END print "...", NR, avg ' <infile

The NR value is incrementing on each record/line awk reads from the input file, means that when awk read all lines the value of NR is the line number of the last line (basically it points to the Record Number)

You almost make it.

enclose compute line in like

BEGIN ...
 compute goes here
END printf "..."

why triple print ?

every assignation like foo=$3+foo is a positive test to awk, and default action is to print line.

(and by the way, you only need to compute average in END clause)