On transaction of purchase TDS provisions are not applicable. TDS U/s 194C is deductible only on AMC Charges.
Here AMC charges paid 24000/- i.e. less than 30000/-
If you pay further AMC in respect of other 2 computers also to the same vendor for a single bill of 48000/ - then AMC charges will be deducted as this will constitute a single payment exceeding30000/- , TDS will be deducted on48000/-. and not on72000 as the limit of75000/- has not yet crossed.
Do not give more THAN 3000/-AMC to this person now , otherwise on exceeding the total AMCs to 75000/- TDS is required to be deducted on 24000/- also WHICH WAS EXEMPT in the first instance earlier.