#Python, 15 bytes

Python 2 (15)

exec"\150elp()"

It concatenates the strings uses the octal 150 which is h and "elp()" and runs the result. This prints the help() command which says "Welcome to Python 3.5's help utility!", meeting the requirements.

Python 3 (17)

exec("\150elp()")

Try it online!

Python, 15 bytes

Python 2 (15)

exec"\150elp()"

It concatenates the strings uses the octal 150 which is h and "elp()" and runs the result. This prints the help() command which says "Welcome to Python 3.5's help utility!", meeting the requirements.

Python 3 (17)

exec("\150elp()")

Try it online!

#Python 2/3, 28 17 (3) and 15 (2) bytes

##Python 3

exec("\150elp()")

It concatenates the strings uses the octal 150 which is h and "elp()" and runs the result. This prints the help() command which says "Welcome to Python 3.5's help utility!", meeting the requirements.

##Python 2

This uses exactly the same method as the above but without the parentheses

exec"\150elp()"

Try it online

#Python, 15 bytes

Python 2 (15)

exec"\150elp()"

It concatenates the strings uses the octal 150 which is h and "elp()" and runs the result. This prints the help() command which says "Welcome to Python 3.5's help utility!", meeting the requirements.

Python 3 (17)

exec("\150elp()")

Try it online!

#Python 2/3, 28 17 (3) and 15 (2) bytes

##Python 3

exec("\150elp()")

It concatenates the strings uses the octal 150 which is h and "elp()" and runs the result. This prints the help() command which says "Welcome to Python 3.5's help utility!", meeting the requirements.

##Python 2

This uses exactly the same method as the above but without the parentheses

exec"\150elp()"

Try it online

#Python 2/3, 28 17 (3) and 15 (2) bytes

##Python 3

exec("\150elp()")

It concatenates the strings uses the octal 150 which is h and "elp()" and runs the result. This prints the help() command which says "Welcome to Python 3.5's help utility!", meeting the requirements.

##Python 2

This uses exactly the same method as the above but without the parentheses

exec"\150elp()"

Try it online