Formula for multiple conditions only returns values for first condition

I am trying to fix a formula and it's driving me mad.

I have a spreasheet with several tabs. On one tab I have a list of names with the values 1 or 2 alongside them.

On another tab I have a set of values for each entry.

Tab 1
Ben 1
Bob 2
Tom 1
Sam 3

Tab 2
Ben 3 4 6
Bob 3 5
Tom 4 5 7
Sam 4

I need the formula to calculate the 3rd-1st value on tab 2 when the tab 1 value is 1, the 3rd-2nd value on tab 2 when the tab 1 value is 2 and to show no value when the tab 1 value is 3.

I've so far got:

=IF('Assessment Results'!$D$6=1,'Progress Data (1 Term)'!G19-'Progress Data (1 Term)'!E19,IF('Assessment Results'!$D$6=2,'Progress Data (1 Term)'!G19-'Progress Data (1 Term)'!F19,IF('Assessment Results'!$D$6=3,'Progress Data (1 Term)'!G19-'Progress Data (1 Term)'!G19)))

But it only works for the first condition. Any help greatly appreciated as I am fairly new to excel and am trying to teach myself. :-)