It's possible, that MongoDB doesn't start if you use the Trusty packages:
Failed to start mongod.service: Unit mongod.service failed to load: No such file or directory.
In this case, install the Debian Wheezy packages as described at the end.
Install the Trusty DEB packages
Download the DEB files
cd
wget http://repo.mongodb.org/apt/ubuntu/dists/trusty/mongodb-org/3.0/multiverse/binary-amd64/mongodb-org-mongos_3.0.3_amd64.deb
wget http://repo.mongodb.org/apt/ubuntu/dists/trusty/mongodb-org/3.0/multiverse/binary-amd64/mongodb-org-server_3.0.3_amd64.deb
wget http://repo.mongodb.org/apt/ubuntu/dists/trusty/mongodb-org/3.0/multiverse/binary-amd64/mongodb-org-shell_3.0.3_amd64.deb
wget http://repo.mongodb.org/apt/ubuntu/dists/trusty/mongodb-org/3.0/multiverse/binary-amd64/mongodb-org-tools_3.0.3_amd64.deb
wget http://repo.mongodb.org/apt/ubuntu/dists/trusty/mongodb-org/3.0/multiverse/binary-amd64/mongodb-org_3.0.3_amd64.deb
And install with (in this order)
sudo dpkg -i mongodb-org-server_3.0.3_amd64.deb
sudo dpkg -i mongodb-org-tools_3.0.3_amd64.deb
sudo dpkg -i mongodb-org-shell_3.0.3_amd64.deb
sudo dpkg -i mongodb-org-mongos_3.0.3_amd64.deb
sudo dpkg -i mongodb-org_3.0.3_amd64.deb
And taken from the comments, thank you @Pilot6:
They may have dependencies to each other. That's why installation
packages one by one may give errors. I suggest sudo dpkg -i mongodb*.deb
Use the Trusty PPA
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
echo "deb http://repo.mongodb.org/apt/ubuntu trusty/mongodb-org/3.0 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.0.list
sudo apt-get update
sudo apt-get install -y mongodb-org
Use the Debian Wheezy packages
It's possible, that MongoDB doesn't start if you use the Trusty packages:
Failed to start mongod.service: Unit mongod.service failed to load: No such file or directory.
In this case, install the Debian Wheezy packages as described below
sudo apt-get remove mongodb-org
sudo apt-get autoremove
sudo rm /etc/apt/sources.list.d/mongodb-org-3.0.list
sudo echo "deb http://repo.mongodb.org/apt/debian wheezy/mongodb-org/3.0 main" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.0.list
sudo apt-get update
sudo apt-get install -y mongodb-org