For Turkey - as a Lebanese, you are allowed visa free access upto 90 days within 6 months period for tourism only.
For the other two, it should be possible to enter Bulgaria using Romanian short stay
C visa or vice versa. There is no clarity about number of entries required on the visa, so trying this with a "Single" entry visa may be risky.
As states waiting to be included in EU, the controlling decision currently in force is Decision No 565/2014/EU. As per Article 3 (emphasis added)
- If Bulgaria, Croatia, Cyprus or Romania decide to apply Article 2, they may, in addition to the documents referred to in that Article, recognise as equivalent to their national visas for transit through or intended stays on their territory not exceeding 90 days in any 180-day period:
national short-stay visas and national long-stay visas issued by Bulgaria, Croatia, Cyprus, or Romania in the uniform format laid down by Council Regulation (EC) No 1683/95 (16);
As per information with EU, both Romania and Bulgaria have implemented this decision:
BULGARIA implements Decision No 565/2014/EU, and in accordance with Article 3 of the Decision recognises national visas and residence permits issued by Croatia, Cyprus and Romania listed, respectively, in Annex II, III and IV of the Decision, as equivalent to Bulgarian visas.
ROMANIA implements Decision No 565/2014/EU, and in accordance with Article 3 of the Decision recognises national visas and residence permits issued by Bulgaria, Cyprus and Croatia listed, respectively, in Annex I, II and III of the Decision, as equivalent to Romanian visas.
Indeed Timatic also agrees
Passengers with a C visa issued by Croatia, Cyprus, Romania or a Schengen Member State valid for the period of intended stay. They are visa exempt for a maximum stay of 90 days.The maximum stay is granted within 180 days.
Conditions for crossing via land should be same as I can't find anything to the contrary.
You have not stated why you are unwilling/unable to obtain a Bulgarian visa but are willing to do so from Romania. If you have been refused a visa by Bulgaria, then there may be complications if you try to enter even using Romanian visa.