There are two different original Japanese equips that are both translated as 'Mythril Armlet(s)'. The first is ミスリルの腕輪 [mythril no udewa] or 'mythril armlet(s)' in Japanese. The second is 精銀の小手 [seigin no kote] or 'spirit silver gauntlet(s)'.
Final Fantasy III

Stats: DEF 2, MDEF 4, INT +1, MND +1, weight 0
Use: FRE, WHM, BLM, RDM, SCH, GEO, EVO, BRD, DEV, MAG, SAG, SMN
Buy: 120 gil (sell: ? gil)
Shop: Kazus, Canaan
Treasure: Tozus
Type: Arms
Other: as 'Mythril Bracers'
Final Fantasy VII

Stats: DEF 18, DEF% 3, MDEF 8
Slots: 2, Growth: Normal
Buy: 350 gil (Sell: ? gil)
Shop: Wall Market, Junon, Kalm
Drop: Heli Gunner
Other: ミスリルの腕輪 in Japanese
Final Fantasy IX
Stats: EVA +17, MEVA +11, SPR +1
Equip: Zidane, Vivi, Garnet (Dagger), Quina, Eiko, Amarant
Abilities: Bug Killer
Buy: 500 gil (sell: 250 gil)
Shop: Cleyra (Burmecian soldier Dan's shop), Lindblum (Dragoos' shop (disc 2), Dragon's Gate merchant (disc 2)), Conde Petie (Thomas Goldpiler's shop), Black Mage Village (equipment shop (disc 3)), Alexandria (weapon shop (disc 3)), Esto Gaza (equipment shop (before Desert Palace)), Daguerreo (equipment shop (disc 3))
Steal: Thorn
Treasure: Pinnacle Rocks jump off a ledge in deepest area
Use: Thief Gloves
Description: Armlet made of mythril
Type: Wrist
Other: ミスリルの腕輪 in Japanese
Record Keeper

Mythril Armlet (VII)
Stats: DEF +6, MDEF +9, ATK +3, MAG +3, MND +3, EVA 96 (lv1), ? (lv5), ? (lv10), DEF +18, MDEF +25, ATK +8, MAG +8, MND +8, EVA 96 (lv15)
Max Level: lv5 (initial), lv10 (first evolve), lv15 (second evolve)
Type: Armlet, Rarity: ★★
Other: ミスリルの腕輪 in Japanese

Mythril Armlet (X)
Stats: Death/Slow resist (moderate), Max Level: 1
Type: Accessory, Rarity: ★★★★★
Other: 精銀の小手 in Japanese
Category: Equipment