In the B737, does an IAN NPA require that the G/P is 3° or can the G/P angle vary?
The G/P can deviate from the typical 3 degrees, but not much.
For any non-precision approach (NPA) flown with VNAV or Integrated Approach Navigation (IAN), the navigation database needs to contain a stored glide path (GP) angle or a suitable waypoint (runway or MAP):

VNAV should be used only for approaches that have one of the following features:

a published GP angle on the LEGS page for the final approach segment
an RWxx waypoint coincident with the approach end of the runway
a missed approach waypoint before the approach end of the runway, (e.g.,
MXxx).

[...]
The FMC
obtains the GP angle displayed on the LEGS page from the navigation database. [...] A GP angle
is coded in the navigation database for nearly all straight-in approach procedures.
[...]
This GP angle is normally defined by the regulatory authority responsible for the
approach procedure and provides a continuous descent at a constant flight path
angle for a final approach path that complies with minimum altitudes at
intermediate step-down fixes. The typical GP angle is approximately 3.00°, but
can vary from 2.75° to 3.77°.

(Boeing 737 NG FCTM 5.45 - Approach and Missed Approach, emphasis mine)
As you can see, the GP angle can vary between 2.75 and 3.77 degrees. For IAN, this G/P information is passed on to the AFDS by the FMC. The list of IAN restrictions does not limit the GP angle:

IAN Requirements and Restrictions

airplanes must be equipped with FMC U10.5 or later and IAN FMA
displays
dual or single engine approaches are authorized
waypoints in the navigation database from the FAF onward may not be
modified
raw data monitoring is required during localizer based approaches.
During FMC based non-ILS approaches, raw data monitoring is
recommended when available in accordance with the techniques
described in the Non-ILS approach section in this chapter
QFE operation is not authorized
cold temperature altitude corrections are not permitted
RNP appropriate for the approach must be used
the autopilot is required until suitable visual reference is established when
performing an approach requiring an RNP of 0.15 or lower.


(Boeing 737 NG FCTM 5.60 - Approach and Missed Approach)
The FCTM does however says the following about when an approach can be flown with IAN:

For FMC based approaches, a proper series of legs/waypoints describing the
approach route including an appropriate vertical path or glide path (GP) angle
must appear on the LEGS page. A GP angle displayed on the LEGS page means
the vertical path complies with final approach step-down altitudes (minimum
altitude constraints). A glide path angle suitable for an IAN approach is one that
approximates 3° and crosses the runway threshold at approximately 50 feet.

(Boeing 737 NG FCTM 5.61 - Approach and Missed Approach, emphasis mine)
It does not say if the full range from 2.75 to 3.77 degrees is considered approximately 3.00°, but it certainly does not have to be exactly 3.