Hm. Well, one problem with low fantasy is that you have to spend time adjusting the default rules as the PHB assumes a high magic setting.
In a low magic setting, spellcasters become *much* more powerful relative to the general population since they're the ones making critical adventuring gear, in possession of powerful healing magic, and the ability to manipulate reality.
That's true to an extent. Even if spells top out at 3rd level, if only those casters can achieve those effects, they're going to be seen as powerful.
There are, however, a lot of options you can do individually or collectively to bring magic more in line.
(1) In order to learn any spell, you must be taught. It doesn't matter if you're a wizard, sorcerer, cleric, druid, bard, paladin, ranger, etc. To learn any spell, you must be shown by someone willing to teach you. Then, you must make a check (caster level + ability mod., or Spellcraft, or Knowledge (arcana/religion), etc.) to learn the spell. Maybe something like DC 20 + twice spell level. You cannot retry, ever. If you want something softer, each retry increases the DC by 5-10.
(2) Spells take longer to recharge. Each spell slot takes a number of hours to recover equal to the spell level. A level 1 comes back after 1 hour, while a level 8 takes 8 hours of rest. If this doesn't seem like enough, make it 1 hour for a level 1, and double each spell level thereafter (2 hours for a level 2, 4 hours for a level 3, 8 hours for a level 4, etc.).
(3) Using a scroll, wand, staff, etc. uses two spell slots of the appropriate level. This still gives the versatility of items, though it keeps them from dominating the game if someone wants to use them all the time.
(4) Level 0-3 spells take 1 round to cast. Level 4-6 spells take 2 rounds to cast. Level 7 or higher spells take 3 rounds to cast. No spell is cast as a standard action, ever (certain exceptions, such as Feather Fall being a free action, can be exempted if the GM wants).
(5) Increase Concentration check DCs to "plus twice spell level" rather than "plus spell level."
(6) Cap spell slots at some point, such as no spell slots above 3rd level. You can gain access to a new level as normal. For example, gaining a level 3 spell slot while leveling up gives you a level 3 spell slot. However, gaining a level 4 spell slot only allows you to cast level 4s, it does not grant a level 4 spell slot. It costs multiple spell slots to cast higher level spells. A level 4 would require a level 3+1, or 2+2. A level 5 would cost a level 3+2, or 2+2+1, or 3+1+1, etc.
(7) The number of spellcaster levels you have cannot exceed your other class levels. This means that certain spells will inherently not be available until very high level, such as Raise Dead (level 18), or Plane Shift (level 18). Other spells are still only available deep into the campaign, such as Fly (level 10) or Fireball (level 10).
(8) Casting any spell makes you fatigued until you have 8 hours of rest. Casting a second spell causes you to become exhausted until you have 1 hour of rest, at which time you become fatigued. Casting a third spell causes you to become unconscious for 9 hours, at which time the exhaustion and fatigue are gone.
(9) You can only cast spells if the environment favors it. No casting Cone of Cold in a volcano, or Create Water while in a desert. Things like Summon spells would be limited to where there are a lot of creatures, such as in cities.
(10) Casting any spell deals damage to you equal to twice spell level. This damage must heal naturally, and cannot be magically healed, and traits like Fast Healing or Regeneration do not apply.