You could use entirely the test functionality to achieve what you want. From the man page of test:

 ! expression  True if expression is false.
 expression1 -a expression2
               True if both expression1 and expression2 are true.
 expression1 -o expression2
               True if either expression1 or expression2 are true.
 (expression)  True if expression is true.

So your condition could look like:

if [ -f file1 -a -f file2 -a -f file3 ] ; then
    # do stuff with the files
fi

For negating use escaped parentheses:

if [ ! \( -f file1 -a -f file2 -a -f file3 \) ] ; then
    # do stuff with the files
fi

You could use entirely the test functionality to achieve what you want. From the man page of test:

 ! expression  True if expression is false.
 expression1 -a expression2
               True if both expression1 and expression2 are true.
 expression1 -o expression2
               True if either expression1 or expression2 are true.
 (expression)  True if expression is true.

So your condition could look like:

if [ -f file1 -a -f file2 -a -f file3 ] ; then
    # do stuff with the files
fi

For negating use escaped parentheses:

if [ ! \( -f file1 -a -f file2 -a -f file3 \) ] ; then
    echo "Error: You done goofed."
    exit 1
fi
# do stuff with the files