These 3 candidates are hidden in the game files. Here is how to access them:
Step1: Go to The Political Machine 2008\Data\English and open your candidates.xml file in notepad.
Step2: Search for "ChloeSullivan" or "FDR"
Edit the section that says:
<!--
<Candidate InternalName="ChloeSullivan">
<FirstName>Chloe</FirstName>
<LastName>Sullivan</LastName>
<Image>gfx\\candidate_images\\Female01.png</Image>
<Config>HillaryClinton</Config>
<Gender>1</Gender>
<FundRaisingAbility>5</FundRaisingAbility>
<Stamina>9</Stamina>
<PoliticalParty>0</PoliticalParty>
<HomeState>9</HomeState>
<Money>3000</Money>
<IssueStanceMap InternalName="Default">
<ALTENERGY>10</ALTENERGY>
<_CHARISMA>7</_CHARISMA>
<_COMELINESS>10</_COMELINESS>
<_CREDIBILITY>6</_CREDIBILITY>
<_EXPERIENCE>2</_EXPERIENCE>
<_INTELLIGENCE>8</_INTELLIGENCE>
<_MEDIABIAS>7</_MEDIABIAS>
<_MINORITYAPPEAL>5</_MINORITYAPPEAL>
<_RELIGIOUS>2</_RELIGIOUS>
</IssueStanceMap>
</Candidate>
-->
To
<Candidate InternalName="ChloeSullivan">
<FirstName>Chloe</FirstName>
<LastName>Sullivan</LastName>
<Image>gfx\\candidate_images\\Female01.png</Image>
<Config>HillaryClinton</Config>
<Gender>1</Gender>
<FundRaisingAbility>5</FundRaisingAbility>
<Stamina>9</Stamina>
<PoliticalParty>0</PoliticalParty>
<HomeState>9</HomeState>
<Money>3000</Money>
<IssueStanceMap InternalName="Default">
<ALTENERGY>10</ALTENERGY>
<_CHARISMA>7</_CHARISMA>
<_COMELINESS>10</_COMELINESS>
<_CREDIBILITY>6</_CREDIBILITY>
<_EXPERIENCE>2</_EXPERIENCE>
<_INTELLIGENCE>8</_INTELLIGENCE>
<_MEDIABIAS>7</_MEDIABIAS>
<_MINORITYAPPEAL>5</_MINORITYAPPEAL>
<_RELIGIOUS>2</_RELIGIOUS>
</IssueStanceMap>
</Candidate>
For FDR the steps are the same.
Sullivan and FDR use existing appearances of Hillary and Romney respectively.
Lieberman is not in the candidates.xml file, so you will have to create a custom candidate with your own stats.
His apperance can be found in: \The Political Machine 2008\Data\Configs JoeLieberman.candidatecfg.xml (this file can be opened in Notepad)