Where would these respective presidents be on the political spectrum?

Lyndon B. Johnson, Gerald Ford, and Barack Obama.

Most presidents govern as moderates.

In a hundred years, they will all be Zachary Tylers and Millard Filmores except Obama will be remembered as the first black president.

Literally, in 100 years people will know Obama was black and perhaps that Johnson escaped the Vietnam war.

I do not know much about Ford and where he stood but he appeared to be on the conservative side,

Obama would be close to the middle but left leaning,

Johnson - far left

It's 3, 7 and 5.
The first two being slightly before the last.

LBJ is a firm leftist and Obama is an admitted socialist. They will be in about the same place on the spectrum, several steps to the left of center. The difference is that LBJ spent his lifetime building a political career while Obama entered the White House with nearly zero experience. LBJ was able to leverage his political capital to accomplish his Great Society. For a particularly disgusting story on LBJ, read about the Silver Star he "earned" in WW2. Obama took the fewest meetings in history with policy makers. His lack of experience is reflected on how little was accomplished during his eight years in office.

Ford… Not elected to the Presidency… Was a moderate, often at odds with fellow republicans regarding social issues. That feuding went on even after Ford left office. Ford was an "insert" after Agnew and then Nixon resigned from office. The republicans needed a centrist that could perform damage control. Ford would be a centrist, perhaps one step to the right of center.