Convincing others of Ruby over Python and PHP
If your goal is to show why language X is better than language Y, you're stuck in subjective-land where there are no right answers.
No, Ruby is not better than PHP or Python. It might be more suited for a given purpose, and for that you can give specific examples. PHP is a poor choice for writing an SMTP server; Ruby and Python will serve you better (in fact, in Python it can be done in just a few lines, can't speak for Ruby though). On the other hand, PHP is better suited than Ruby for writing a one-off, short backend for an email submission form. The code is short, easy to maintain, quick to write, etc.
PHP has an absolutely huge developer base, making programmers easy to find, which comes in handy if you ever want to out-source any aspect of the development chain. On the other hand, it's also terrifically easy to write horrible code in PHP, and there is more than enough of that going around.
Python has a much larger user base than Ruby, and indeed is the primary language that RedHat uses for developing system tools. So if you're on a RedHat derived server (and statistically, chances are pretty good that you are if you're using Linux) then Python is guaranteed to be already in-place and working properly, etc.
In short, weigh the benefits, make a decision, but don't assume that people will agree with you; after all it's just an opinion.

Edit
It just occurred to me that I failed to state the whole point: you shouldn't be trying to convince other people that they should use Ruby over Python/PHP. Instead you should be trying to determine whether you should use Ruby over Python/PHP. 
You can't go fact-finding like this having already determined what the answer will be -- that's not helpful. Instead you should be gathering information on the benefits and drawbacks of each language and weighing that against the requirements of your company. Once you come to a conclusion, you'll already have a preponderance of evidence showing it was the correct one.
G'day folks. I'm trying to introduce Ruby at work, and a few people are interested. However, I've been asked to present the benefits of Ruby over Python and PHP.
I've broken this down into 2 parts:
1) show Python and Ruby's advantages over PHP;
2) show Ruby's advantages over Python.
The first is easy. I'll explain things like:

Everything's an object.
Python and Ruby are easier to read and write.

For the second, I'm thinking of:

Ruby has many conveniences, which makes it easier to read and write. Eg: Optional brackets, and being able to open built-ins, allows for things like 2.days.from_now
RSpec is miles ahead of Python's TDD and BDD frameworks.
GitHub and RubyForge are fantastic resources for finding, releasing, and collaborating on software.

Do you have any suggestions? I'm all ears!