The third person in the line is the President Pro Tempore of the Senate (normally the most senior US senator).
The succession is as follows:
Vice President
Speaker of the House of Representatives
President Pro Tempore of the Senate
He/she isn’t second, but third.
Clarify the presidential line of succession
Speaker of a house.
It specifies the full line of succession should the president no longer be able to serve. The Vice-President has always been second in line, but the Act specifies the next 18 places, including the Speaker of the House in 3rd, President pro tempore of the Senate in 4th, and on down the line of Cabinet Secretaries.
the Cabinet
Originally, congress was to decide who was next in line for office after the Vice President. The 25th Amendment changed presidential succession to what it is today.
the Cabinet
Secretary of state
presidential succession is the order in which the office of president is to be filled
He is third in line, just behind the Speaker of the House.
Third in line, after vice-president and speaker of the house.
The senate pro tempore is third in the line of presidential succession, behind the vice-president and the Speaker of the House. ( I hope this is what you want to know.)