Firstly: 

There is nothing wrong with a worshipper reciting two or three soorahs after al-Faatihah, whether he is an imam or a member of the congregation, except that the imam should do that only occasionally to show that it is permissible, and without making the prayer too long for the people praying behind him. It is also permissible to do that both in obligatory and naafil prayers. 

It is proven that the Prophet (peace and blessings of Allaah be upon him) recited two soorahs after al-Faatihah in many prayers, and these soorahs are known as “pairs”. They are mentioned in several saheeh ahaadeeth. 

It was narrated that ‘Amr ibn Murrah said: I heard Abu Waa’il say: A man came to Ibn Mas’ood and said: I recited al-Mufassal last night in one rak’ah. He said, “That is too quick, like reciting poetry. I know the ‘pairs’ that the Prophet (peace and blessings of Allaah be upon him) used to recite together,” and he mentioned twenty soorahs from al-Mufassal, two soorahs in each rak’ah.

Narrated by al-Bukhaari, 742; Muslim, 822. 

Al-Bukhaari included this hadeeth in a chapter which he called, “Reciting two soorahs in one rak’ah.” 

Al-Mufassal is that portion of the Qur'aan which goes from Qaaf to al-Naas. 

It was narrated that ‘Ilqimah and al-Aswad said: A man came to Ibn Mas’ood and said: “I recite al-Mufassal in one rak’ah.” He said, “That is too quick, like reciting poetry. But the Prophet (peace and blessings of Allaah be upon him) used to recite ‘pairs’, two soorahs in one rak’ah: al-Najm and al-Rahmaan in one rak’ah, al-Qamar and al-Haaqqah in one rak’ah, al-Toor and al-Dhaariyyaat in one rak’ah, al-Waaqi’ah and al-Qalam in one rak’ah, sl-Ma’aarij and al-Naaz’iaat in one rak’ah, al-Mutaffifoon and ‘Abasa in one rak’ah, al-Muddaththir and al-Muzzammil in one rak’ah, al-Insaan and al-Qiyaamah in one rak’ah, al-Naba’ and al-Mursalaat in one rak’ah, al-Dukhaan and al-takweer in one rak’ah.

Narrated by Abu Dawood (1396); classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

When praying qiyaam al-layl, the Prophet (peace and blessings of Allaah be upon him) recited al-Baqarah, al-Nisa’ and Aal ‘Imraan in a single rak’ah, as was narrated by Hudhayfah (may Allaah be pleased with him), as narrated by Muslim, 772. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: 

It is permissible for a person to recite two or three soorahs after al-Faatihah, or he may limit himself to just one, or he may divide a soorah in two halves (split between two rak’ahs). All of these are permissible, because of the general meaning of the verse in which Allaah says (interpretation of the meaning): 

“So, recite you of the Qur’aan as much as may be easy for you”

[al-Muzzammil 73:20]

And the Prophet (peace and blessings of Allaah be upon him) said: “Then recite whatever you can of the Qur’aan.” 

Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 13/question no. 500 

Shaykh al-Albaani (may Allaah have mercy on him) said: 

The Prophet (peace and blessings of Allaah be upon him) would sometimes recite two or more soorahs in one rak’ah. 

Sifat Salaat al-Nabi ( (peace and blessings of Allaah be upon him), p. 103-105. The Shaykh (may Allaah have mercy on him) quotes the ahaadeeth which have been narrated concerning that, along with their isnaads.

[This book by Shaykh al-Albaani is available in English under the title The Prophet’s Prayer Described – Translator]. 

Secondly: 

With regard to reciting Soorat al-Ikhlaas after every recitation in every rak’ah, it is narrated that one of the Sahaabah did that and the Prophet (peace and blessings of Allaah be upon him) approved of it. This is among the evidence concerning the basic question, which is reciting two soorahs in each rak’ah. 

It was narrated from ‘Aa’ishah that the Prophet (peace and blessings of Allaah be upon him) sent a man in charge of a raiding party, and he used to lead his companions in prayer, always ending his recitation with Qul Huwa Allaahu ahad. When they returned, they mentioned that to the Prophet (peace and blessings of Allaah be upon him), and he said, “Ask him why he does that.” They asked him and he said, “Because it is a description of the Most Merciful, and I love to recite it.” The Prophet (peace and blessings of Allaah be upon him) said: “Tell him that Allaah loves him.” Narrated by al-Bukhaari, 6940; Muslim, 813). 

This hadeeth indicates that it is permissible to do this, although it is not mustahabb because the Prophet (peace and blessings of Allaah be upon him) did not do it or persist in doing it, and the best of guidance is the guidance of Muhammad (peace and blessings of Allaah be upon him). 

And Allaah knows best.