A rather simple quick way that explains itself:
cat FileName | sed 's/[0-9]*//g' | sed 's/\<boy\>//g' | sed 's/\<girl\>//g' | sort -u | wc -l
Or to satisfy @αғsнιη's concern about  UUoC:
<FileName sed 's/[0-9]*//g' | sed 's/\<boy\>//g' | sed 's/\<girl\>//g' | sort -u | wc -l
Or another UUoC compliant command:
sed 's/[0-9]*//g' <FileName | sed 's/\<boy\>//g' | sed 's/\<girl\>//g' | sort -u | wc -l
A notice to @Rebi Khalifa:
@αғsнιη rightly wrote in the comments below:
or <fileName cut -d' ' -f3 |sort -u |wc -l; cat filename | ... is
  UUoC
@steeldriver rightly wrote in the comments below:
I'd suggest using cut rather than all those sed commands - you
  should at least combine them into a single invocation ex. sed -E -e
  's/^[0-9]+//' -e 's/\b(boy|girl)\b//'
They both used field selection approach which is the same approach you were trying to implement to solve your issue based on what you wrote in your question:
I tried with awk to access only the third column but I can't get it to
  print the number of lines.
One does not need to be sophisticated to get things done in Ubuntu! Things can be done in many unimaginable ways.
One way which praises the KISS principle is to pipe | simple commands one to the next until mission is accomplished:
- Read the content of the file with cat FileName-->
- Pipe it |-->
- Remove number groups with sed 's/[0-9]*//g'-->
- Pipe it |-->
- Remove the word boy with sed 's/\<boy\>//g'-->
- Pipe it |-->
- Remove the word girl with sed 's/\<girl\>//g'-->
- Only names are left now... -->
- Pipe it |-->
- Sort the names and remove duplicates with sort -u-->
- Only unique names ar left now -->
- Pipe it |-->
- Count the lines with wc -l-->
- Done