It is not HUF property 

on father demise property would devolve on his legal heirs 

No this not fall under HUF property. 

It will be treated as your father's self acquired property. 

Therefore upon his intestate death,  the property shall devolve equally on all his legal heirs as successors in interest. 

As per the HUF, The property which are inherited and comes to legal heirs do come under HUF automatically. Yes, eldest son will be Karta. but other legal heirs should accept this property as HUF to continue further, if they are asking for partition then it will not called as HUF further.

- Since, your father was the sole son , and hence he was having his right over the inherited house individually , and further he purchased the another house from selling the said house and also taken loan , then after his demise it will not come under the huf property. 

- Hence, after his demise , this property would be devolved upon all his heirs equally , i.e. you three brother having 1/3rd each share in the property , if there is no other legal heirs alive. 

1. Whether you and your two brothers alongwith your respective families were living together (joint family), when your father (Karta) was alive?. If so, then this will be considered as HUF property and in such a situation you could be the Karta of this HUF.

2.  If the property were to be treated as self acquired property owned by your deceased father, then on his intestate death, the property would devolve equally to your mother (if alive) and their children (including daughters). In the instant case, assuming that your mother is not alive and you have no sister and only two brothers, each one of you will be entitled to 1/3rd undivided share in the property.

After death of father, property in the hands of his children becomes ancestral property, more so the property purchased from proceeds of sale of ancestral property. 

This is not a HUF property. It was his property and it will go to his legal heirs. Since you are his son you are one of the legal heirs. You may form a HUF and then the Property may become a HUF property.

If the said property is formed under huf then yes 

Typically, the karta is the eldest coparcener of an HUF who acts as the head and is responsible for managing its legal and fi nancial affairs. 3. A junior coparcener of the HUF can be appointed as the karta if all the coparceners and members give their consent.

The question of HUF property arises only if it is a joint Hindu family which has been in common possession and enjoyment of all the coparceners for generations together and has not been partitioned. Your father's estate is not ancestral as you say he had inherited it (as his absolute and exclusive share). After your father's death, his estate devolves upon all his legal heirs equally, you and your brothers, sisters, mother and grandmother (father's mother). You cannot be legally recognised as a 'karta'.