Each method has its requirement to give best results.
Each measure is highlighted in green if the measurement is reliable and in orange or red if not.

First method may give inaccurate measures indoor, or with poor sky visibility (inside narrow canyon, near high buildings). Even heavy clouds may reduce accuracy.
Anyway it may gives errors of 20m and more even with best conditions.

Second method may give inaccurate measures if it cannot locate you with precision. Be sure you have enabled GPS and Wifi positioning.
It may also fail when you are near a drop-off (canyon or ravine). A small positioning error can give large measurement error.

Third method will give bad results if it is not able to identify pressure from nearest airport. It cannot do that if it cannot locate the device or without internet connection.
Measure may be also poor if you are distant from the airport or when weather is changing quickly.
Another reason of inaccuracy it's the deviation of the barometric sensor of your device. It can be corrected performing calibration (only on paid version of the App).