Rabelais: Are you sure? Maybe you meant only the 3.0 psionic prestige classes, which are kinda useless; however, the 3.0 Soulknife is still pretty good (but aimed more toward rogue/psion or rogue/psychic warrior multiclass characters).
OP: First, I would recommend the following: if using 3E psionics, it may be best to impose a limit on how many power points a psionic character can access at any given time; limit it to 1/3rd their maximum, for instance, or maybe, just maaaaybe, 1/4th their maximum (but always rounded up, else it would screw up 1st-level psionic characters too badly). Say that a psionic character can shift points into their 'available pool' through 1 minute (or perhaps 10 minutes) of uninterrupted meditation (but the available pool still cannot exceed the aforementioned limit of 1/3rd or 1/4th their total PP).
That will prevent them from spending all their points in one or two fights each day. It isn't necessary, but is important if players intend to go full-bore in every encounter and then insist that everyone rest for a day after each battle, just so the psionic ones can replenish. This is only a problem with munchkins and rude players, though, thus why it isn't always necessary. This is just a suggested houserule for use sometimes.
Also: a very important point to make sure you're aware of. Both 3E versions of psionics are very similar and have a limit on how many power points can be spent on any given power manifestation. If I recall correctly, the maximum you can spend on a power is equal to your manifester level (so a 10th-level psion can only spend up to 10 power points on any given manifestation of a power). I can't remember where this is noted exactly in each book, but it is in both the PsiHB and the XPH; I think the maximum was a bit different in 3.0, but I can't seem to find the entry right now. Just keep the power-points-per-manifestation limit in mind.
Generally though, 3e psionics is much less complicated than 2E, and usually less abusable (there are still a few problem powers, but they at least have some limitations and easy means to houseruling them to be fixed). 3.0 psionics has the one complication of psionic combat modes; they're simpler than in 2E, but still present. 3.5 changes them into normal psionic powers that have a simple effect and doesn't distinguish them as special or anything; for instance, Mind Thrust in the 3.5 Expanded Psionics Handbook just deals 1d12 damage (with a Will save allowed to negate the damage altogether), and has an augmentation to deal some more damage if you spend more points on it.
Psionics are not really overpowered unless you allow every power to be used as-written and ignore any errata; there are just a few powers that really need tweaking, especially in 3.0 (3.5 psionics has a few more problematic bits).
In 3.0 the only power I can recall at the moment that really needs modding is Psychofeedback; just add a limit to its description that says "You cannot spend more power points than your manifester level each round on Psychofeedback's benefits (separately from the power's own cost)." Psychofeedback is used generally to 1) boost Str bonus for one round to get a huge attack/damage increase, 2) boost an ability score bonus for one round to make power save DCs virtually impossible to overcome (excepting a natural 20 by the defender), or 3) boost defenses for a round when really needed (spending 40 pp on a Dexterity bonus means +20 AC for one round, which could make you unhittable, barring natural 20s). Severely limiting the power point expenditure per round will solve the problem (it's a very inefficient power, just very strong if you let a PC/NPC blow most/all their power points on it in a single round).
Psychofeedback was altered in 3.5 psionics and is now seriously limited, but does have the dubious addition of being able to boost Int, Wis, and Cha modifiers, not just physical ability modifiers. However, 3.5 adds some more problem powers; Astral Construct is generally quite superior to Summon spells in 3.5 (I forget if it's a little better than Summons in 3.0, but the difference in 3.5 is greater IIRC); some of the Energy blast powers of 3.5 are too powerful in their augmentations; the book titled "Complete Psionic", from Wizards of the Coast, includes some errata to fix this problem, if I recall correctly, but the only errata it has for Astral Construct is, IIRC, just a limitation on having just one Astral Construct active at a time. A few other powers have similar scaling problems to the Energy powers. Otherwise the augmentations are reasonable.
Also, there's one or two other powers in 3.5 I think that need to be removed or tweaked; basically the ones that give extra attacks or actions per round, like Form of Doom, IIRC. Schism is a bit different in 3.5, but probably still too powerful (mainly because Haste was nerfed in 3.5 to only give an extra attack, not an extra partial action in general). Hustle, however, is probably not broken; 3.5 made it provide just an extra move action, and you can only use it once per round (it's manifested like a Quickened spell, as a 'swift action'), which is quite meager for a 2nd-level power (3.5 bumped it up a level).
There's a feat or something else in Lords of Madness, I think, that is only available to aberrations; it is part of a sick combo with Elans from the Expanded Psionics Handbook, and you should probably just change the Elan race's type to 'magical beast', which would solve that problem. Similarly, there is a combo with the prestige class called Warshaper, or maybe it was the Master of Many Forms (or perhaps it was both), that is nasty when combined with psionics; just add some restriction that bars those from psionic characters (like "must not possess a power point reserve"). I think these few things are the only major problems with 3.5 psionics; there may be a few lesser problems but these are the only ones I've seen mentioned or complained about before.