Significance of the Agni's idol with two heads seven hands and three legs

 There was no Idol worship in Rig Vedic era.

This idol relates, perhaps, to the concept of Rig Vedic verse 4.58.3 and this idol might have been created in some Hundreds of years back, to depict this Rig Vedic idea about AGNI.

चत्वारि शृङ्गा त्रयो अस्य पादा द्वे शीर्षे सप्त हस्तासो अस्य । त्रिधा बद्धो वृषभो रोरवीति महो देवो मर्त्याँ आ विवेश ॥
catvāri śṛṅgā trayo asya pādā dve śīrṣe sapta hastāso asya | tridhā baddho vṛṣabho roravīti maho devo martyām̐ ā viveśa ||

English translation:

“Four are his horns; three are his feet; his heads are two, his hands are seven; the triple-bound showerer (of benefits) roars aloud; the mighty deity has entered among men.”

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As the Rig Veda is very difficult to understand, unless a SPIRITUALLY elevated person helps, only a guesswork can be done, to decipher its meaning.

AGNI is eulogised as BRAHMAN, ie., the formless ENERGY that governs the Universe.

Rig Veda eulogises AGNI as BRAHMAN. It also uses various epithets, ie., INDRA, VISHNU, VARUNA, ADITI, SAVITAR, etc, indicating the BRAHMAN.

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In order to realise the BRAHMAN, 7 seers will help the spiritual aspirant. Hence, the 7 seers might have been depicted as 7 hands.

Three feet indicates that AGNI is all pervasive and thus can be found on EARTH, ATMOSPHERE and in the HEAVEN.

Rig Veda 1.61.9

अस्येदेव प्र रिरिचे महित्वं दिवस्पृथिव्याः पर्यन्तरिक्षात् । स्वराळिन्द्रो दम आ विश्वगूर्तः स्वरिरमत्रो ववक्षे रणाय ॥
asyed eva pra ririce mahitvaṃ divas pṛthivyāḥ pary antarikṣāt | svarāḻ indro dama ā viśvagūrtaḥ svarir amatro vavakṣe raṇāya ||

Just this one—his greatness projected beyond heaven and earth, beyond the midspace. Sovereign king, Indra, sung by all in the house, a reverberant tankard, waxed strong for battle [/for joy].

2 Heads might indicate that BRAHMAN is the combination of MASCULINE as well as FEMININE ENERGIES, depicted by AGNI and ADITI, respectively.

Rig Veda 1.24.2

अग्नेर्वयं प्रथमस्यामृतानां मनामहे चारु देवस्य नाम । स नो मह्या अदितये पुनर्दात्पितरं च दृशेयं मातरं च ॥
agner vayam prathamasyāmṛtānām manāmahe cāru devasya nāma | sa no mahyā aditaye punar dāt pitaraṃ ca dṛśeyam mātaraṃ ca ||

English translation:

“Let us invoke the auspicious name of Agni, the first divinity of the immortals, that he may give us to the great Aditi, and that I may behold agin my father and my mother.”

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I repeat this is only a guesswork.