Try this:
awk '/Nmap scan report/ { host=$NF } NF==4 && $2=="open" { print host, $2, $NF }' nmap-output.txt
Explanation:
1) For any line that matches the string "Nmap scan report", remember the last field of the line in the variable name "host"
2) For any line that has 4 fields and where the second field matches the string "open", print the remembered "host" variable, the second field of this line ("open"), and the last field of the line (what is in the Version column).