Changing directions during potty training takes more time and patience than sticking to one course from the beginning. If your puppy is 3 months old her control over elimination is rapidly improving and her ability to postpone elimination until the right location is close. You could stick to your present course knowing that the frequency of outdoor visits will decrease steadily. Probably placing the tray with a pad near the door that she goes out, in a small play pen so that you can indicate that it is time to potty but then place her in the pen and use the same verbal clues and praise that you use outdoors.
You could also consider a covered dog pen constructed outside a doggy door. If anchored to the ground and smartly placed this could offer enough security for her. Check with suppliers of dog kennels in your area or fencing companies.