Assessment of Adjunctive Cannabidiol Oral Solution (GWP42003-P) in Children With Tuberous Sclerosis Complex (TSC), Dravet Syndrome (DS), or Lennox-Gastaut Syndrome (LGS) Who Experience Inadequately-controlled Seizures

Key Inclusion Criteria:

* Participants with TSC (1 month to \< 2 years of age), or DS (1 year to \< 2 years of age), or LGS (1 year to \< 2 years of age) within the specified age range at the time of initial informed consent.
* Parent(s)/legal representative is/are willing and able to give informed consent for participation in the study.
* Parent(s)/legal representative is/are willing and able (in the investigator's opinion) to comply with all study requirements (including accurate electronic participant-reported outcome \[ePRO\] diary completion).
* Participants with TSC must have a diagnosis per the 2012 International Tuberous Sclerosis Complex Consensus Conference. Participants with LGS or DS must have a diagnosis that is consistent with International League Against Epilepsy (ILAE) guidelines and confirmed by the Epilepsy Study Consortium (ESCI).
* Participants who have uncontrolled seizures, and who are currently receiving 1 or more antiseizure medication (ASMs).
* A suitable VEEG, as available in the medical record, within 1 year of Visit 1. When a historical VEEG is not available, and if clinically indicated and appropriate (due to uncertainties or new seizures), a VEEG will be completed and read to confirm diagnosis prior to Visit 3. All VEEGs are to be read at baseline by the investigator and by an independent reviewer.
* Has seizures which are not adequately controlled through their current ASMs, defined as ≥ 1 seizure reported on the seizure diary during the screening/baseline period

Key Exclusion Criteria:

* Has tumor growth which, in the opinion of the investigator, could affect participant safety.
* Has clinically significant abnormal laboratory values, in the investigator's opinion, at screening/baseline.
* Has clinically significant abnormalities in the electrocardiogram (ECG) measured at screening/baseline.
* Has any concurrent cardiovascular conditions, that will, in the investigator's opinion, interfere with the ability to assess their ECGs.
* Has any known or suspected hypersensitivity to cannabinoids or any of the excipients of the study intervention such as sesame seed oil.
* Has significantly impaired hepatic function prior to Visit 3, defined as:

  * Serum alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \> 3 × upper limit of normal (ULN) and (total bilirubin \[TBL\] \> 2 × ULN or international normalized ratio \[INR\] \> 1.5).
  * Serum ALT or AST \> 5 × ULN.
  * Serum ALT or AST \> 3 × ULN with the presence of fatigue, nausea, vomiting, right upper quadrant pain or tenderness, fever, rash, and/or eosinophilia (\> 5%).
  * Elevated ALT or AST should be discussed with the medical monitor prior to Visit 3; the medical monitor may allow for a confirmatory re-draw prior to Visit 3.
* Has received another study intervention within 4 weeks prior to Visit 1 or plans to take another study intervention during the study.
* Has any other clinically significant disease or disorder which, in the opinion of the investigator, may either put the participant, other participants, or site staff at risk because of participation in the study, may influence the result of the study, or may affect the participant's ability to take part in the study.
* Any clinically significant abnormalities identified following a physical examination of the participant that, in the opinion of the investigator, would jeopardize the safety of the participant if they took part in the study.
* Has previously been enrolled into this study.
* Has plans to travel outside their country of residence during the study, unless the participant has confirmation that the study intervention is permitted in the destination country.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.