Hi. Congrats on your new lo.
The first thing that caught my eye was that you mentioned she really starts to get tired after 1 hour from waking in the morning. This is totally normal, and for most babies, the first period of A time in the day is the shortest. I actually went through a situation when someone recommended to me to put my dd down for her first nap 1 hour after waking in the morning. The thought is that after being well rested from the night's sleep, baby will be better able to settle and sleep well for the nap. Then, after she hopefully has a good first nap, the other naps following will be better since sleep begets sleep. I know you said that she wakes happy at 45 mins, but that happiness doesn't seem to be carrying through the whole A time. I don't mean to suggest that she's not a happy baby, just that she's tired. Anyway, if she does continue to have short naps, you will need to monitor her A times accordingly. Meaning, you will need to watch her sleep cues after the next feeding and not try to keep her up 1.5-2 hrs after each feeding since she's already had some of her A time before the feeding. Overtiredness will lead to poor naps.
As for extending her feeds to 3.5 hrs, I would do it gradually. Perhaps try to lenghten one feed time at a time. Each feeding doesn't have to be spaced exactly 3 hrs apart. If you notice that there is a consistancy in which feed she takes less at, lengthen that one for a day or two and see if she starts to eat more. Hopefully, once you find a good pattern for feeding during the day, she'll take more and maybe stop waking at night. However, night wakings are not uncommon at her age, so don't fret. Also, 3 months old is a prime time for a growth spurt, and if she's not taking it during the day, she'll be looking for it during the night (or even if she did eat alot during the day she would still wake at night during a growth spurt).
I hope some of this helps. What is her temperment, by the way?