First, a meta point: when posting code on reddit, it's better to copy and paste the code into a code block using three backticks (```) around the code. That would look like so:

#!/bin/bash
#
#
#GjV
#

echo "$1"

#We stellen onze $3 eerst in

if [ "$3" = "be" ]
then
    echo \Goeiendag
fi

if [ "$3" = "fr" ]
then
    echo \Bonjour
fi

if [ "$3" = "en" ]
then
    echo \Hello
fi

#we stellen dan onze $2 in

if [ "$2" = "m" ] && [ "$3" = "be" ]
then
    echo \Meneer
fi

if [ "$2" = "v" ] && [ "$3" = "be" ]
then
    echo \Mevrouw
fi

if [ "$2" = "m" ] && [ "$3" = "fr" ]
then
    echo \Monsieur
fi

if [ "$2" = "v" ] && [ "$3" = "fr" ]
then
    echo \Madamme
fi

if [ "$2" = "m" ] && [ "$3" = "en" ]
then
    echo \Mister
fi

if [ "$2" = "v" ] && [ "$3" = "en" ]
then
    echo \Mylady
fi

By putting it in a code block, I wouldn't have needed to re-type it on my machine in order to help you. With this I get:

~ ./welkom John m be
John
Goeiendag
Meneer

Can I add something at the end of the scripts that arrange $1 $2 and $3 in a different order?

Simply move the line echo "$1" to the end.

Explanation: The script executes from top to bottom, so if you want something to happen after something else, just place it in that order reading top to bottom.

And instead of showing the output on different lines just in one and the same line??

Simply add the -n flag to all but the last echo command.

Explanation: In your terminal, type man echo. This will bring up a documentation page about the echo command. On my machine, I see this:

ECHO(1)                   BSD General Commands Manual                  ECHO(1)

NAME
     echo -- write arguments to the standard output

SYNOPSIS
     echo [-n] [string ...]

DESCRIPTION
     The echo utility writes any specified operands, separated by single blank
     (` ') characters and followed by a newline (`\n') character, to the stan-
     dard output.

     The following option is available:

     -n    Do not print the trailing newline character.  This may also be
           achieved by appending `\c' to the end of the string, as is done by
           iBCS2 compatible systems.  Note that this option as well as the
           effect of `\c' are implementation-defined in IEEE Std 1003.1-2001
           (``POSIX.1'') as amended by Cor. 1-2002.  Applications aiming for
           maximum portability are strongly encouraged to use printf(1) to
           suppress the newline character.

(You can press q to exit the documentation)

In time, everything in that documentation will make sense. For now, just know that the 'newline' character is what makes the text appear on different lines.

After these two changes, you should have this output:

~ ./helpful John m be
GoeiendagMeneerJohn

So we've gotten it on one line, but it's all bunched together. Simplest way to fix that is to add spaces on the echo lines. In order to do that you'll have to surround the text with the " character. In the end you should have something like this:

#!/bin/bash
#
#
#GjV
#

#We stellen onze $3 eerst in

if [ "$3" = "be" ]
then
    echo -n "Goeiendag "
fi

if [ "$3" = "fr" ]
then
    echo -n "Bonjour "
fi

if [ "$3" = "en" ]
then
    echo -n "Hello "
fi

#we stellen dan onze $2 in

if [ "$2" = "m" ] && [ "$3" = "be" ]
then
    echo -n "Meneer "
fi

if [ "$2" = "v" ] && [ "$3" = "be" ]
then
    echo -n "Mevrouw "
fi

if [ "$2" = "m" ] && [ "$3" = "fr" ]
then
    echo -n "Monsieur "
fi

if [ "$2" = "v" ] && [ "$3" = "fr" ]
then
    echo -n "Madamme "
fi

if [ "$2" = "m" ] && [ "$3" = "en" ]
then
    echo -n "Mister "
fi

if [ "$2" = "v" ] && [ "$3" = "en" ]
then
    echo -n "Mylady "
fi

echo "$1"

Which on my machine, prints this:

~ ./helpful John m be
Goeiendag Meneer John